som_alerts_dispatcher: Handle failed build look ups.

Change 5465002a "Add mst Android PFQ Configuration for master-arc-dev
Android branch" broke SoM dispatcher because it was searching for a
master build which does not exist until there is a waterfall restart.

BUG=chromium:804375
TEST=som_alerts_dispatcher

Change-Id: I9c9b0fa56f2af0326d614f3348689c52b099b846
Reviewed-on: https://chromium-review.googlesource.com/879107
Commit-Queue: David Riley <davidriley@chromium.org>
Tested-by: David Riley <davidriley@chromium.org>
Trybot-Ready: David Riley <davidriley@chromium.org>
Reviewed-by: Jacob Kopczynski <jkop@chromium.org>
1 file changed