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/common/error_test.cpp

@@ -7,9 +7,31 @@
 #include "gtest/gtest.h"
 
 namespace Carbon {
+namespace {
 
 TEST(ErrorTest, FatalUserError) {
-  ASSERT_DEATH({ FatalUserError() << "test"; }, "ERROR: test\n");
+  ASSERT_DEATH({ FATAL_RUNTIME_ERROR_NO_LINE() << "test"; }, "ERROR: test\n");
 }
 
+TEST(ErrorTest, FatalRuntimeError) {
+  ASSERT_DEATH({ FATAL_RUNTIME_ERROR_NO_LINE() << "test"; },
+               "RUNTIME ERROR: test\n");
+}
+
+TEST(ErrorTest, FatalCompilationError) {
+  ASSERT_DEATH({ FATAL_COMPILATION_ERROR_NO_LINE() << "test"; },
+               "COMPILATION ERROR: test\n");
+}
+
+TEST(ErrorTest, FatalUserErrorLine) {
+  ASSERT_DEATH({ FATAL_USER_ERROR(1) << "test"; }, "ERROR: 1: test\n");
+}
+
+auto NoReturnRequired() -> int { FATAL_USER_ERROR_NO_LINE() << "test"; }
+
+TEST(ErrorTest, NoReturnRequired) {
+  ASSERT_DEATH({ NoReturnRequired(); }, "ERROR: test\n");
+}
+
+}  // namespace
 }  // namespace Carbon