Fix CassandraDatabaseSchemaEditor.create_model (#100)

Thanks for the fix! It was probably my mistake, nice catch!

django_cassandra_engine/base/schema.py

@@ -20,7 +20,7 @@ def create_model(self, model):
                 MigrationSchemaMissing as Exc)
         except ImportError:
             from django.core.exceptions import ImproperlyConfigured as Exc
-        raise Exception('No schema for cassandra database')
+        raise Exc('No schema for cassandra database')
 
     def delete_model(self, model):
         pass