buildbots: better handle empty/nonexistent gm-expectation directories
buildbots: better handle empty/nonexistent gm-expectation directories

This will allow us to revert https://code.google.com/p/skia/source/detail?r=9642
('skimage: if --readPath points at an empty dir, create an empty expectations file')
if we like...

R=borenet@google.com

Review URL: https://codereview.chromium.org/17303004

git-svn-id: 32fc27f4dcfb6c0385cd9719852b95fe6680452d@9645 2bbb7eff-a529-9590-31e7-b0007b416f81

Tiger66639/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,Tiger66639/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,Tiger66639/skia-buildbot,google/skia-buildbot,Tiger66639/skia-buildbot,Tiger66639/skia-buildbot,Tiger66639/skia-buildbot,google/skia-buildbot,Tiger66639/skia-buildbot