Use an HttpResponseBadRequest to return HTTP error

Even with status=400 we were seeing 200 responses coming back from
nested module updating. Everywhere else we use HttpResponseBadRequest
to indicate a 400.

Fixes #264
Increase size of other_notes field

This limit was somewhat abitrary and was included to deter people
from writing too much.

However, people want to write more. Increase the limit.
Merge pull request #266 from alphagov/increase-text-field-size

Increase size of other_notes field
Merge pull request #265 from alphagov/bugfix/bad-request-on-nested-module-error

Use an HttpResponseBadRequest to return HTTP error