DM: dump images for task failures too (but not skipped tasks)

This will get us back to uploading things like serialize-8888 mismatches.

BUG=skia:4806
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1614443002

Review URL: https://codereview.chromium.org/1614443002
1 file changed