Switch away from the deprecated Field.error_message attribute.

Review Request #10205 — Created Oct. 8, 2018 and submitted — Latest diff uploaded

Information

Review Board
release-4.0.x
8c95027...

Reviewers

Django deprecated Field.error_message prior to Django 1.6, but we
still had a couple of cases where it was used. This change simply moves
us to the supported error_messages attribute, and cleans up some
surrounding code.

Manually tested that the errors were working.