Report pending errors from MetalCodeGenerator

This mirrors the change in https://skia-review.googlesource.com/c/skia/+/441311,
but for Metal this time.

Change-Id: I1abdb844b6392c81252532f9ff4a91b993edb66e
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/441401
Reviewed-by: John Stiles <johnstiles@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
1 file changed