Don't fail silently when a formula is invalid.
Don't fail silently when a formula is invalid.

BUG=skia:
R=mtklein@google.com, bensong@google.com

Author: jcgregorio@google.com

Review URL: https://codereview.chromium.org/546913002

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