[dot] Tidy up error handling
[dot] Tidy up error handling

Change-Id: I7b88c1962d3f452fe1d4b8deec66b04c0e651d4f
Reviewed-on: https://skia-review.googlesource.com/c/buildbot/+/258236
Auto-Submit: Kevin Lubick <7cdab2cfab351f23814786ba39716e90eed69047@google.com>
Commit-Queue: Joe Gregorio <b9cf2471ff6d33504cafe8e6f356b732b2cadced@google.com>
Reviewed-by: Joe Gregorio <b9cf2471ff6d33504cafe8e6f356b732b2cadced@google.com>

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