Fix crash on flutter update-packages (#28922)

When running git update-packages, if the goldens repo is dirty, the
flutter tool currently crashes and logs the failure from git pull.

This adds a check for a dirty git client and emits a friendlier error
message in the case where it's not clean, and avoid the crash by
catching the NonZeroExitCode exception and rethrowing as toolExit
instead.

Fixes: https://github.com/flutter/flutter/issues/28915