geodjango should return more informative error for lack of MySQL distance queries

[robstar]

A better error should be reported for when a distance lookup is attempted with mysql:

>>> qs = Building.gobjects.filter(geo_loc__distance_lte=(x, D(km=7)))
Traceback (most recent call last):
  File "<console>", line 1, in <module>
  File "/root/django-svn/Django-1.0-beta_2/django/db/models/manager.py", line 90, in filter
    return self.get_query_set().filter(*args, **kwargs)
  File "/root/django-svn/Django-1.0-beta_2/django/db/models/query.py", line 481, in filter
    return self._filter_or_exclude(False, *args, **kwargs)
  File "/root/django-svn/Django-1.0-beta_2/django/db/models/query.py", line 499, in _filter_or_exclude
    clone.query.add_q(Q(*args, **kwargs))
  File "/root/django-svn/Django-1.0-beta_2/django/db/models/sql/query.py", line 1189, in add_q
    can_reuse=used_aliases)
  File "/root/django-svn/Django-1.0-beta_2/django/db/models/sql/query.py", line 1061, in add_filter
    parts, opts, alias, True, allow_many, can_reuse=can_reuse)
  File "/root/django-svn/Django-1.0-beta_2/django/db/models/sql/query.py", line 1377, in setup_joins
    raise FieldError("Join on field %r not permitted." % name)
FieldError: Join on field 'geo_loc' not permitted.

[jbronn]

Yes, a better error needs to be raised.