Explicit error for spec is None

synapse/storage/prepare_database.py

@@ -487,7 +487,10 @@ def _upgrade_existing_database(
                 spec = importlib.util.spec_from_file_location(
                     module_name, absolute_path
                 )
-                assert spec is not None
+                if spec is None:
+                    raise RuntimeError(
+                        f"Could not build a module spec for {module_name} at {absolute_path}"
+                    )
                 module = importlib.util.module_from_spec(spec)
                 spec.loader.exec_module(module)  # type: ignore