[louhi] Handle error and nil DocRef in GetLatestFlowExecutions
[louhi] Handle error and nil DocRef in GetLatestFlowExecutions

Didn't check non-Done error...

Bug: skia:13861
Change-Id: I869ea7afa14171b093d083dbda79efef5431d46a
Reviewed-on: https://skia-review.googlesource.com/c/buildbot/+/599299
Reviewed-by: Ravi Mistry <9fa2e7438b8cb730f96b74865492597170561628@google.com>
Auto-Submit: Eric Boren <0e499112533c8544f0505ea0d08394fb5ad7d8fa@google.com>
Commit-Queue: 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