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/testdata/struct_field_missing.carbon

@@ -0,0 +1,12 @@
+// 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
+
+struct Point {
+  var x: Int;
+  var y: Int;
+}
+
+fn main() -> Int {
+  return Point(.x = 1).x - 1;
+}