[messages] Improve messages_suite error reporting.

Changes the loop in `messages_suite.dart` so that if an exception
occurs while processing a message, the name of the message is included
in the error message. This makes it much easier to debug
`messages_suite` failures.

Change-Id: I6a6a69649348b068b60c44969e74d39086ddee4a
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/453160
Reviewed-by: Konstantin Shcheglov <[email protected]>
Commit-Queue: Paul Berry <[email protected]>

Author: stereotype441