skdiff: return nonzero exit value if there are any mismatches

This will allow us to replace "gm -r" in our buildbots with "skdiff".  This will speed things up, and work around http://code.google.com/p/skia/issues/detail?id=473 ('PDF gradtext gm image results are nondeterministic')
Review URL: https://codereview.appspot.com/6242071

git-svn-id: http://skia.googlecode.com/svn/trunk@4097 2bbb7eff-a529-9590-31e7-b0007b416f81
3 files changed