Don't log stacktrace for UnexpectedTaskStateError

An UnexpectedTaskStateError is actually an expected behavior and we
don't need to log a stacktrace every time it happens. This stacktrace
is one of the most common ones found in nova-compute during gating jobs.

Change-Id: I8b33f72dc0f4beaca969de084481d66987faa974
Merge "Don't log stacktrace for UnexpectedTaskStateError"