handle failing pathop tests

Some tests isolated from GrShapes_arcs do not
fail gracefully, so make sure errors are properly
handled.

TBR=reed@google.com

Bug: skia:
Change-Id: Ia8c9903e64ef755ec11c398df3e5d258ca1f5f8b
Reviewed-on: https://skia-review.googlesource.com/143112
Reviewed-by: Cary Clark <caryclark@skia.org>
Commit-Queue: Cary Clark <caryclark@skia.org>
Auto-Submit: Cary Clark <caryclark@skia.org>
8 files changed