[rbe] Error out if the API gave us an incomplete directory list
[rbe] Error out if the API gave us an incomplete directory list

Change-Id: I8315db9332dd4fad1c580f9954bf496d4b1e323e
Reviewed-on: https://skia-review.googlesource.com/c/buildbot/+/590919
Commit-Queue: Eric Boren <0e499112533c8544f0505ea0d08394fb5ad7d8fa@google.com>
Reviewed-by: Ravi Mistry <9fa2e7438b8cb730f96b74865492597170561628@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