Fix --rewrite-blacklist after adding --exit-unexpected

finishHandler called reportFailure in a never-taken code path before
--exit-unexpected started using err to signal an unexpected result.
This resulted in duplicated reports.

Also, don't allow both --rewrite-blacklist and --exit-unexpected, as it would
produce a short blacklist.

Change-Id: Ie62a6b548929f1642b3cf862d1f44e6c002d590c