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/ast/expression.cpp

@@ -169,9 +169,8 @@ auto Expression::MakeTupleLiteral(int line_num,
   for (auto& arg : args) {
     if (arg.name == "") {
       if (seen_named_member) {
-        FatalUserError()
-            << line_num
-            << ": positional members must come before named members";
+        FATAL_USER_ERROR(line_num)
+            << "positional members must come before named members";
       }
       arg.name = std::to_string(i);
       ++i;