rebaseline.py: add --keep-going-on-failure option, off by default

R=borenet@google.com

Review URL: https://codereview.chromium.org/18092004

git-svn-id: http://skia.googlecode.com/svn/trunk@10109 2bbb7eff-a529-9590-31e7-b0007b416f81
3 files changed