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During a recent service outage, most of our hosts came back online but
some did not.  Looking at the logs, the hosts which failed to come
online were treating HTTP 500 errors as 'UNAUTHENTICATED' which is a
permanent error that caused the hosts to go offline.

Instead of returning UNAUTHENTICATED in this case, UNAVAILABLE should
be returned.  This is a retryable error and is handled as such in the
various classes which use the ProtobufHttpClient class.

Change-Id: Idcfe98082f382b4053dfd6dbb2b4570621375f15
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2595521Reviewed-by: Yuwei Huang <yuweih@chromium.org>
Commit-Queue: Joe Downing <joedow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#837705}