Improve error message for missing migrations in an app

            # so we're fine.

            return

        if key[0] in self.migrated_apps:

            try:

                return list(self.graph.root_nodes(key[0]))[0]

        raise ValueError("Dependency on unknown app %s" % key[0])

            except IndexError:

                raise ValueError("Dependency on app with no migrations: %s" % key[0])

        raise ValueError("Dependency on unknown app: %s" % key[0])

    def build_graph(self):

        """