Make fatal errors if trybot job fails or trybot image is empty.

Update GetTrybotImage to check the final trybot job status and
return an error if the job did not have a success status. Also
update the nightly buildbot test jobs to fail fatally if the
trybot or vanilla image names are empty.

BUG=None
TEST=Tested this in role account.

Change-Id: I17c4ca5fa77ea488c8bf3ae4e5c225b747c90c87
Reviewed-on: https://chrome-internal-review.googlesource.com/196095
Reviewed-by: Yunlian Jiang <yunlian@google.com>
Commit-Queue: Caroline Tice <cmtice@google.com>
Tested-by: Caroline Tice <cmtice@google.com>
2 files changed