rebaseline.py: error out early if --expectations-root not pointing at gm-expected dir
BUG=https://code.google.com/p/skia/issues/detail?id=1403
R=senorblanco@chromium.org

Review URL: https://codereview.chromium.org/18416005

git-svn-id: http://skia.googlecode.com/svn/trunk@9961 2bbb7eff-a529-9590-31e7-b0007b416f81
15 files changed