ModelAdmin should fail gracefully when provided a non-sense modelclass

[maxime-rainville]

If you try to manage to access a ModelAdmin with a non-sensical model (e.g.: http://example.com/admin/my-model-admin/EditForm/field/Acme-Web-Some-Models-That-Doesnt-exists) you get some pretty scary 500 error.

This should return either a 400 or 404 error.

PRs

• FIX return 404 instead of 500 for invalid model class, fixes #570 #1757

[dhensby]

I guess it's a 404.

The reason it's a 500 is because an error is triggered and that's somewhat helpful for users when debugging; I'm not sure we can set the error response code and maintain the error info, though?

[maxime-rainville]

While it is developer friendly, it's not very CMS-user friendly. The live version of the error is not very helpful either.

I think our entire CMS error page set up could be improved. Right now, the 404 error page in-cms just displays an unthemed version of the main error page.