Improve debug information provided when backend format and GrColorType don't match

Bug: 1037949
Change-Id: Iabc380647acf006f2035c5cedff8391cc87b7def
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/262805
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
2 files changed