Changed get_model to raise an exception on errors.

Returning None on errors required unpythonic error checking and was
inconsistent with get_app_config.

get_model was a private API until the previous commit, but given that it
was certainly used in third party software, the change is explained in
the release notes.

Applied the same change to get_registered_model, which is a new private
API introduced during the recent refactoring.

django/db/migrations/state.py

@@ -192,11 +192,12 @@ class ModelState(object):
             meta_contents["unique_together"] = list(meta_contents["unique_together"])
         meta = type("Meta", tuple(), meta_contents)
         # Then, work out our bases
-        bases = tuple(
-            (apps.get_model(*base.split(".", 1)) if isinstance(base, six.string_types) else base)
-            for base in self.bases
-        )
-        if None in bases:
+        try:
+            bases = tuple(
+                (apps.get_model(*base.split(".", 1)) if isinstance(base, six.string_types) else base)
+                for base in self.bases
+            )
+        except LookupError:
             raise InvalidBasesError("Cannot resolve one or more bases from %r" % (self.bases,))
         # Turn fields into a dict for the body, add other bits
         body = dict(self.fields)