Remove RuntimeError / CompilationError. (#2258)

Instead, work out the prefix for an error based on whether it was produced during parsing, semantic analysis, or when running the program.

explorer/interpreter/exec_program.cpp

@@ -18,9 +18,9 @@
 
 namespace Carbon {
 
-auto ExecProgram(Nonnull<Arena*> arena, AST ast,
-                 std::optional<Nonnull<llvm::raw_ostream*>> trace_stream)
-    -> ErrorOr<int> {
+auto AnalyzeProgram(Nonnull<Arena*> arena, AST ast,
+                    std::optional<Nonnull<llvm::raw_ostream*>> trace_stream)
+    -> ErrorOr<AST> {
   if (trace_stream) {
     **trace_stream << "********** source program **********\n";
     for (const auto decl : ast.declarations) {
@@ -54,6 +54,14 @@ auto ExecProgram(Nonnull<Arena*> arena, AST ast,
     for (const auto decl : ast.declarations) {
       **trace_stream << *decl;
     }
+  }
+  return ast;
+}
+
+auto ExecProgram(Nonnull<Arena*> arena, AST ast,
+                 std::optional<Nonnull<llvm::raw_ostream*>> trace_stream)
+    -> ErrorOr<int> {
+  if (trace_stream) {
     **trace_stream << "********** starting execution **********\n";
   }
   return InterpProgram(ast, arena, trace_stream);