TemplateNotFound exception could be a lot more helpful

[boxed]

We should suggest the correct filename. For example today I tried "foo.html" and it should have been "foo/foo.html" ('foo' being unique!). Running over all of the directories and doing some Levenshtein distance guessing should get us very far I think.

[boxed]

Also if you don't have app dirs setting on we could warn about that.

[boxed]

It could find if you've missed putting an app in installed apps.