Switch from llvm::errs()+exit() to FATAL_USER_ERROR() stream-based handling. (#670)

This is intended to:

- Standardize use of line_num (and highlight where no line_num is available).
- Standardize indications of runtime/compilation errors.
- Make it incrementally easier to write new errors.

Co-authored-by: Chandler Carruth <chandlerc@gmail.com>

executable_semantics/interpreter/heap.cpp

@@ -4,6 +4,7 @@
 
 #include "executable_semantics/interpreter/heap.h"
 
+#include "executable_semantics/common/error.h"
 #include "llvm/ADT/StringExtras.h"
 
 namespace Carbon {
@@ -33,9 +34,9 @@ void Heap::Write(const Address& a, const Value* v, int line_num) {
 
 void Heap::CheckAlive(const Address& address, int line_num) {
   if (!alive_[address.index]) {
-    llvm::errs() << line_num << ": undefined behavior: access to dead value "
-                 << *values_[address.index] << "\n";
-    exit(-1);
+    FATAL_RUNTIME_ERROR(line_num)
+        << ": undefined behavior: access to dead value "
+        << *values_[address.index];
   }
 }
 
@@ -44,8 +45,7 @@ void Heap::Deallocate(const Address& address) {
   if (alive_[address.index]) {
     alive_[address.index] = false;
   } else {
-    llvm::errs() << "runtime error, deallocating an already dead value\n";
-    exit(-1);
+    FATAL_RUNTIME_ERROR_NO_LINE() << "deallocating an already dead value";
   }
 }