Better testing of SkCodec to 565

If getPixels returns an unexpected result, do not create a digest
and compare it to digests for premul and unpremul.

Print a better error message, including the name of the image and
string versions of the SkCodec::Result.

Change-Id: Ie98e350a1ef91c0745fe9de0855192e5793e033c
Reviewed-on: https://skia-review.googlesource.com/c/176582
Auto-Submit: Leon Scroggins <scroggo@google.com>
Reviewed-by: Nigel Tao <nigeltao@google.com>
Commit-Queue: Leon Scroggins <scroggo@google.com>
1 file changed