FIXED #12810: return validation error if a registered table name is used for m2m fields

Ticket: https://code.djangoproject.com/ticket/12810

Description: Throw an error with the checks framework when m2m fields are configured to use a table that is already registered.
You could get away with something a bit more compact than that I think -- you don't need to wrap the text in parentheses, and you probably shouldn't split up the class name just to adhere to PEP8, line length isn't a big worry. Same goes for the other instance.

```python
expected = [
    Error(
        "Field is using a table that has already been "
        "registered by <class 'invalid_models_tests.test_models.Bar'>.",
        hint=None, obj=Foo._meta.get_field('bar'), id='fields.E340'
    )
]
```