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.