Fix a crash when attempting to set submitter to an invalid value.

Review Request #10143 — Created Sept. 12, 2018 and submitted — Latest diff uploaded

Review Board

We had some error handling for checking that someone wasn't trying to
set the submitter of a review request to a user that didn't exist, but
this failed in the local site case (and in the case of other failures
such as authentication backend errors). This change reorders the code so
that we don't get an UnboundLocalError, instead returning the correct
webapi error.

This adds a couple unit tests for the two cases (expected and unexpected
errors), and cleans up a related unit test that I came across while I
was in here.

Ran unit tests.