Replaced std::exit() with return Carbon::ErrorOr for expected errors like invalid syntax (#1120)

* Replaced std::exit() with return llvm::Expected/llvm::Error<T> for expected errors like invalid syntax.

* Use llvm::formatv() for formatting lexer error messages.
x

* Addresed merge errors.

* Fixed impl scope.

* Made ErrorBuilder::operator<< nodiscard, to catch code forgetting 'return' in 'return FATAL_COMPILATION_ERROR()'.

* FatalComplationError() -> ParseAndLexContext::RecordLexerError().
Other usages of ERROR_TOKEN in lexer.lpp were actually supposed to be END_OF_FILE.

* Update executable_semantics/syntax/parse_and_lex_context.h

Co-authored-by: Jon Meow <jperkins@google.com>

* Code review fixes.

* Update executable_semantics/syntax/parser.ypp

Co-authored-by: Jon Meow <jperkins@google.com>

* More code review fixes.

* Update executable_semantics/interpreter/type_checker.h

Co-authored-by: Jon Meow <jperkins@google.com>

* Yet more code review fixes...

* Update executable_semantics/syntax/lexer.lpp

Co-authored-by: Jon Meow <jperkins@google.com>

* code review comments

* Update executable_semantics/interpreter/interpreter.cpp

Co-authored-by: Geoff Romer <gromer@google.com>

* Apply suggestions from code review

Co-authored-by: Jon Meow <jperkins@google.com>

* Update executable_semantics/syntax/lexer.lpp

Co-authored-by: Jon Meow <jperkins@google.com>

* code review

* code review

* Apply suggestions from code review

Co-authored-by: Jon Meow <jperkins@google.com>

* formatted code

* review comments

* Switched to the new ErrorOr<V> error implementation

* code review comments

* fixed comment

* restored ostream.h as #976 makes the change unnecesary

* review comments

Co-authored-by: Jon Meow <jperkins@google.com>
Co-authored-by: Geoff Romer <gromer@google.com>

executable_semantics/interpreter/action_stack.cpp

@@ -7,6 +7,7 @@
 #include "executable_semantics/interpreter/action.h"
 #include "llvm/ADT/StringExtras.h"
 #include "llvm/Support/Casting.h"
+#include "llvm/Support/Error.h"
 
 namespace Carbon {
 
@@ -49,7 +50,7 @@ void ActionStack::Initialize(ValueNodeView value_node,
 
 auto ActionStack::ValueOfNode(ValueNodeView value_node,
                               SourceLocation source_loc) const
-    -> Nonnull<const Value*> {
+    -> ErrorOr<Nonnull<const Value*>> {
   if (std::optional<Nonnull<const Value*>> constant_value =
           value_node.constant_value();
       constant_value.has_value()) {
@@ -74,9 +75,9 @@ auto ActionStack::ValueOfNode(ValueNodeView value_node,
       return *result;
     }
   }
-  // TODO: Move these errors to name resolution and explain them more clearly.
-  FATAL_RUNTIME_ERROR(source_loc)
-      << "could not find `" << value_node.base() << "`";
+  // TODO: Move these errors to compile time and explain them more clearly.
+  return FATAL_RUNTIME_ERROR(source_loc)
+         << "could not find `" << value_node.base() << "`";
 }
 
 void ActionStack::MergeScope(RuntimeScope scope) {
@@ -110,7 +111,7 @@ void ActionStack::InitializeFragment(ContinuationValue::StackFragment& fragment,
   fragment.StoreReversed(std::move(reversed_todo));
 }
 
-void ActionStack::FinishAction() {
+auto ActionStack::FinishAction() -> ErrorOr<Success> {
   std::unique_ptr<Action> act = todo_.Pop();
   switch (act->kind()) {
     case Action::Kind::ExpressionAction:
@@ -123,9 +124,11 @@ void ActionStack::FinishAction() {
     case Action::Kind::DeclarationAction:
       PopScopes();
   }
+  return Success();
 }
 
-void ActionStack::FinishAction(Nonnull<const Value*> result) {
+auto ActionStack::FinishAction(Nonnull<const Value*> result)
+    -> ErrorOr<Success> {
   std::unique_ptr<Action> act = todo_.Pop();
   switch (act->kind()) {
     case Action::Kind::StatementAction:
@@ -139,27 +142,33 @@ void ActionStack::FinishAction(Nonnull<const Value*> result) {
       PopScopes();
       SetResult(result);
   }
+  return Success();
 }
 
-void ActionStack::Spawn(std::unique_ptr<Action> child) {
+auto ActionStack::Spawn(std::unique_ptr<Action> child) -> ErrorOr<Success> {
   Action& action = *todo_.Top();
   action.set_pos(action.pos() + 1);
   todo_.Push(std::move(child));
+  return Success();
 }
 
-void ActionStack::Spawn(std::unique_ptr<Action> child, RuntimeScope scope) {
+auto ActionStack::Spawn(std::unique_ptr<Action> child, RuntimeScope scope)
+    -> ErrorOr<Success> {
   Action& action = *todo_.Top();
   action.set_pos(action.pos() + 1);
   todo_.Push(std::make_unique<ScopeAction>(std::move(scope)));
   todo_.Push(std::move(child));
+  return Success();
 }
 
-void ActionStack::RunAgain() {
+auto ActionStack::RunAgain() -> ErrorOr<Success> {
   Action& action = *todo_.Top();
   action.set_pos(action.pos() + 1);
+  return Success();
 }
 
-void ActionStack::UnwindTo(Nonnull<const Statement*> ast_node) {
+auto ActionStack::UnwindTo(Nonnull<const Statement*> ast_node)
+    -> ErrorOr<Success> {
   while (true) {
     if (const auto* statement_action =
             llvm::dyn_cast<StatementAction>(todo_.Top().get());
@@ -169,24 +178,30 @@ void ActionStack::UnwindTo(Nonnull<const Statement*> ast_node) {
     }
     todo_.Pop();
   }
+  return Success();
 }
 
-void ActionStack::UnwindPast(Nonnull<const Statement*> ast_node) {
-  UnwindTo(ast_node);
+auto ActionStack::UnwindPast(Nonnull<const Statement*> ast_node)
+    -> ErrorOr<Success> {
+  RETURN_IF_ERROR(UnwindTo(ast_node));
   todo_.Pop();
   PopScopes();
+  return Success();
 }
 
-void ActionStack::UnwindPast(Nonnull<const Statement*> ast_node,
-                             Nonnull<const Value*> result) {
-  UnwindPast(ast_node);
+auto ActionStack::UnwindPast(Nonnull<const Statement*> ast_node,
+                             Nonnull<const Value*> result) -> ErrorOr<Success> {
+  RETURN_IF_ERROR(UnwindPast(ast_node));
   SetResult(result);
+  return Success();
 }
 
-void ActionStack::Resume(Nonnull<const ContinuationValue*> continuation) {
+auto ActionStack::Resume(Nonnull<const ContinuationValue*> continuation)
+    -> ErrorOr<Success> {
   Action& action = *todo_.Top();
   action.set_pos(action.pos() + 1);
   continuation->stack().RestoreTo(todo_);
+  return Success();
 }
 
 static auto IsRunAction(const Action& action) -> bool {
@@ -194,7 +209,7 @@ static auto IsRunAction(const Action& action) -> bool {
   return statement != nullptr && llvm::isa<Run>(statement->statement());
 }
 
-void ActionStack::Suspend() {
+auto ActionStack::Suspend() -> ErrorOr<Success> {
   // Pause the current continuation
   todo_.Pop();
   std::vector<std::unique_ptr<Action>> paused;
@@ -205,6 +220,7 @@ void ActionStack::Suspend() {
       llvm::cast<const ContinuationValue>(*todo_.Top()->results()[0]);
   // Update the continuation with the paused stack.
   continuation.stack().StoreReversed(std::move(paused));
+  return Success();
 }
 
 void ActionStack::PopScopes() {