Add informative error message for missing errcheck in presubmit
Add informative error message for missing errcheck in presubmit

TBR=humper
BUG=skia:3623

Review URL: https://codereview.chromium.org/1055803002

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