Fixed error that failed to display user-not-found text in red error box.

When non-existant people or groups were entered into the review request target
reviewers fields, the error box didn't show the text correctly.

Testing done:
1. Incorrectly inputted into Group field - error appeared with text.
2. Incorrectly inputted into People field - error appeared with text.

Fixes bug 2869.
Reviewed at http://reviews.reviewboard.org/r/3756/
Merge branch 'release-1.7.x'