Add a throw statement for imgtestAdd non 0 exit codes.

testing/skia_gold_client/lib/skia_gold_client.dart

@@ -257,11 +257,20 @@ class SkiaGoldClient {
     final ProcessResult result = await _runCommand(imgtestCommand);
 
     if (result.exitCode != 0) {
-      // We do not want to throw for non-zero exit codes here, as an intentional
-      // change or new golden file test expect non-zero exit codes. Logging here
-      // is meant to inform when an unexpected result occurs.
-      print('goldctl imgtest add stdout: ${result.stdout}');
-      print('goldctl imgtest add stderr: ${result.stderr}');
+final StringBuffer buf = StringBuffer()
+        ..writeln('Skia Gold received an unapproved image in post-submit ')
+        ..writeln('testing. Golden file images in flutter/engine are triaged ')
+        ..writeln('in pre-submit during code review for the given PR.')
+        ..writeln()
+        ..writeln('Visit https://flutter-engine-gold.skia.org/ to view and approve ')
+        ..writeln('the image(s), or revert the associated change. For more ')
+        ..writeln('information, visit the wiki: ')
+        ..writeln('https://github.com/flutter/flutter/wiki/Writing-a-golden-file-test-for-package:flutter')
+        ..writeln()
+        ..writeln('Debug information for Gold --------------------------------')
+        ..writeln('stdout: ${result.stdout}')
+        ..writeln('stderr: ${result.stderr}');
+      throw Exception(buf.toString());
     } else if (verbose) {
       print('stdout:\n${result.stdout}');
       print('stderr:\n${result.stderr}');