Don't display empty error messages.
Don't display empty error messages.

BUG=skia:
R=rmistry

Review-Url: https://codereview.chromium.org/2151333002

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