[1.7.x] Revert "Improved AppRegistryNotReady message."

This reverts commit 154f5f0d.

Aymeric: "I chose not to talk about django.setup() here on purpose.
This will hardly always be the correct solution."

Backport of 8121860b from master

django/apps/registry.py

@@ -128,7 +128,7 @@ class Apps(object):
         Raises an exception if all models haven't been imported yet.
         """
         if not self.models_ready:
-            raise AppRegistryNotReady("Models aren't loaded yet. Ensure django.setup() has been called.")
+            raise AppRegistryNotReady("Models aren't loaded yet.")
 
     def get_app_configs(self):
         """