SkShaper: better error info.

Change-Id: Ie7966e22282ef25861cc775bd0904c113ded76c1
Reviewed-on: https://skia-review.googlesource.com/c/171523
Commit-Queue: Hal Canary <halcanary@google.com>
Commit-Queue: Ben Wagner <bungeman@google.com>
Auto-Submit: Hal Canary <halcanary@google.com>
Reviewed-by: Ben Wagner <bungeman@google.com>
1 file changed