Better error messages for nonexistent CCs, reviewers, etc. in arc

Summary:
The existing message is confusing and might cause people to think
that there is something wrong in the field name, e.g., 'CC' instead
of the value.

Test Plan:
Tested by putting random crap in CC and Reviewers and made sure the
error message was using the new format.

Reviewed By: jungejason
Reviewers: epriestley, jungejason
CC: mdevine, jungejason
Differential Revision: 145