Print a warning and continue if no results could be fetched in rebaseline_from_try_jobs.py.

For example, if there's a network issue or results are at some point
not archived in the expected place, a warning can be printed here
instead of exiting with a AttributeError (NoneType has no method...).

BUG=474273

Review-Url: https://codereview.chromium.org/2102963003
Cr-Commit-Position: refs/heads/master@{#403034}
1 file changed