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
flutter_tools cannot depend on flutter_goldens (as flutter_goldens
depdends on the Flutter sdk), so this commit splits client.dart from
flutter_goldens to a pure-dart flutter_goldens_client package.
