[autoroll] Don't log spurious 404s from Gitiles
[autoroll] Don't log spurious 404s from Gitiles

If there's an actual problem we'll get a "failed to transition" alert.

Change-Id: I9e201f1a779a52169e2e6058029369513bcbe9c9
Reviewed-on: https://skia-review.googlesource.com/c/buildbot/+/388816
Reviewed-by: Ravi Mistry <9fa2e7438b8cb730f96b74865492597170561628@google.com>
Commit-Queue: Eric Boren <0e499112533c8544f0505ea0d08394fb5ad7d8fa@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