Improve test that error codes were generated
Improve test that error codes were generated

It appears that invoking exit with a value other than either zero (0) or
254 causes the test infrastructure to interpret the failure as a failure
in the infrastructure, turning the bot purple rather than red. Thowing
an exception is interpreted correctly, so I believe that this change will
cause a need to regenerate to show up as a red bot.

Change-Id: I01e9fc2bfe09fac3da2a9aade69fabf3abd17179
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/216260
Reviewed-by: Paul Berry <72d414e3a99fb120308437779d51667bcc0984eb@google.com>
Commit-Queue: Brian Wilkerson <1f7641b6b14c52b9163524ab8d9aabff80176f21@google.com>

dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk