Add error msg to SkMesh factories

Bug: b/260252882
Change-Id: I5f396028748effc56b7fe62717be40aa5ade65dd
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/612438
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
4 files changed