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/syntax/parse.cpp

@@ -23,8 +23,8 @@ auto parse(const std::string& input_file_name)
     -> std::variant<AST, SyntaxErrorCode> {
   yyin = fopen(input_file_name.c_str(), "r");
   if (yyin == nullptr) {
-    FatalUserError() << "Error opening '" << input_file_name
-                     << "': " << std::strerror(errno);
+    FATAL_USER_ERROR_NO_LINE() << "Error opening '" << input_file_name
+                               << "': " << std::strerror(errno);
   }
 
   std::optional<AST> parsed_input = std::nullopt;