Each element in the errors value is now a dict, with a msg field and an
id field. The id field contains a value out of errors.py that can be
used by the front end to key on. The msg field is the same as what's
been there.
The idea is to keep the number of IDs somewhat limited so there's not a
huge number of things for the front end to know.
(cherry picked from commit 9677b012da)