Add a FatalUserError macro to help print user-caused errors. (#668)

executable_semantics/common/error_test.cpp

@@ -0,0 +1,15 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+#include "executable_semantics/common/error.h"
+
+#include "gtest/gtest.h"
+
+namespace Carbon {
+
+TEST(ErrorTest, FatalUserError) {
+  ASSERT_DEATH({ FatalUserError() << "test"; }, "ERROR: test\n");
+}
+
+}  // namespace Carbon