Arcyd: handle unknown reviewers more gracefully

Now that we can detect the 'unknown reviewers' parse error specifically,
we don't need to treat it as a fatal error and can instead display a
helpful warning on the review page.

Add 'test_unknown_reviewer' case to acceptance_tests.sh

Test Plan:
$ ./precommit.sh
$ ./testbed/arcyd/acceptance_tests.sh

exercise unknown reviewer and self reviewer with
$ ./testbed/arcyd/test_shell.sh