skdiff: add --failonmismatches and --listfilename options, plus cleanup

These changes are needed in order to switch the buildbots from using "gm -r" to "skdiff" to compare gm results, and should be generally good for humans too.
Review URL: https://codereview.appspot.com/6392054

git-svn-id: http://skia.googlecode.com/svn/trunk@4579 2bbb7eff-a529-9590-31e7-b0007b416f81
11 files changed