Fixed #26440 -- Added system checks to verify urlpatterns contains valid objects.

[burhan]

https://code.djangoproject.com/ticket/26440

1. Adding new tests for "fake" objects in URLs
2. Adding new tests for case where user forgets to type url, and ends up with a tuple (r'', foo, name=df)
3. Added new warnings.

[timgraham]

This message should include some helpful information to debug the error. Currently it gives no information as to where the problem is. This is the idea behind the existing describe_pattern() function.

[knbk]

I'm a bit hesitant about exposing the private details of url resolving in user-facing messages, so I would remove the comment about a custom object. Custom url objects are an advanced and unsupported use-case (for now), and I think this warning should target the mistakes made when upgrading/using the new structure, not the ones made when replacing the internal and private details of url resolving.

[timgraham]

+1

[timgraham]

Also, the tests aren't passing, e.g.:

  File "/home/tim/code/django/django/core/checks/urls.py", line 84, in check_pattern_startswith_slash
    "forgotten to add 'url' to your pattern.".format(describe_pattern(pattern)),
  File "/home/tim/code/django/django/core/checks/urls.py", line 40, in describe_pattern
    description = "'{}'".format(pattern.regex.pattern)
AttributeError: 'str' object has no attribute 'pattern'

[alasdairnicol]

This can be closed now that #6514 is merged.