Handle submission errors more gracefully

[janine9vn]

Description

Show an error message on the form if the server responds with an unhandled error code during submission.

Handling of common errors such as malformed or missing input is already covered, but other unexpeted errors should be accounted for, as the forms currently load forever with no indication of error to the end-user.

Implementation

Add a check for the result of the submission post on the form page, and handle it as needed. There should be enough details for the user to have a clear course of action. Some errors such as timeouts would probably have the user retry the request, while others might ask the user to contact us.