Change error message when delete has multiple collections.

Author: WaVEV

django_mongodb/query.py

@@ -66,7 +66,7 @@ def __repr__(self):
     def delete(self):
         """Execute a delete query."""
         if self.compiler.subqueries:
-            raise NotSupportedError("Subqueries are not supported in delete operations.")
+            raise NotSupportedError("Cannot use QuerySet.delete() when a subquery is required.")
         return self.collection.delete_many(self.mongo_query).deleted_count
 
     @wrap_database_errors