Default error views shouldn't silence TemplateDoesNotExist errors if a custom template name was specified

[Simon Charette]

Default error views attempt to load a template from a default name and fallback to a string value if a TemplateDoesNotExist error is raised in the process.

This behavior should be altered to actually raise the error if a custom template_name is specified.

The affected views are:

1. django.views.defaults.page_not_found
2. django.views.defaults.server_error
3. django.views.defaults.bad_request
4. django.views.defaults.permission_denied

Note that the views were identified during a preliminary search and it's possible this pattern is used elsewhere in the Django code base.

We should consider deprecating this behavior instead of simply turning it on since it's slightly backward incompatible.

Thanks to Raphael Michel at #DUTH for helping identifying this issue.

[Carl Meyer]

Yes, I think we'd need a deprecation path here where initially we raise a PendingDeprecationWarning on an explicitly-specified nonexistent template, then a DeprecationWarning, and then finally switch to raising the TemplateNotFound.

[Tim Graham]

I'm lukewarm on deprecations (since it adds a lot of overhead for us) instead of making minor backwards incompatible changes in cases like this where it seems to simply delay revealing probable errors in user code. A similar case was select_related() field name validation where we made a backwards-incompatible change -- Carl, I believe you said you thought that should have gone through a deprecation. Just wanted to bring it up for discussion so I can crystallize my thinking around changes like this. Thanks!

[Tim Graham]

No one indicated an opposition ​on the mailing list to skipping a deprecation and making this a backwards incompatible change.

[Iacopo Spalletti]

Ok, I'll change the PR to avoid the deprecation period and edit the changelog the reflect this change

[Tim Graham <timograham@…>]

In 21bf685:

Fixed #25697 -- Made default error views error when passed a nonexistent template_name.