Update formValidate to use $setViewValue

This seems to be the correct way to interact with Angular views.
Reset "response" error when input changes

In the formValidate directive, previously the error would only be reset
once the form was resubmitted, this caused confusing behaviour.

Fixes #1375.
Merge pull request #1379 from hypothesis/1375-fix-server-errors

Fix server errors in login form
Fix #1375