Improve error handling if form step is invalid

Problem/Motivation

You get this message if the MigrateUpgradeForm has an invalid value for the form 'step'.

This was queried by catch in #47 of #2922701: Migrate UI - refer to modules and add help text

How do we end up here? Should it not throw an AccessDeniedHttpException or something? I wouldn't know what to do if I got this error message except try to navigate back to the beginning of the form via URL hacking or something.

Proposed resolution

Decide what to do
Implement the decision

Remaining tasks

User interface changes

Yes, error message at /upgrade.