Add friendly error pages

[David Biddle]

This should include:
404 for page not found
500 for unexpected errors
Service unavailable (in case we need to take the service down temporarily)
Notify error (special case of the 500 error with some extra text about the submission failing)

[David Biddle]

I've run into an issue here:

• forms-runner uses /form_id routes to navigate to a form, where form_id is an integer
• Rails wants to use /404 and /500 for error pages
  So there will be a conflict if/when form_id is 404 or 500.

I can try writing some more custom logic for handling the error, or we can change the path where forms are hosted, but since we already have a task in the current sprint to look at the URLs forms live at, it might be easier and more opportune to change the form route instead.

[Thomas Iles]

Yes saving the / might make our lives easier in the future if we ever want to add more pages in there.

[David Biddle]

For now at least I'll remove the path: "/" from the forms entry in routes.rb, so forms will now be at /form/1 and pages will now be at /form/1/1.