Instantiate DeferredTimedOutError correctly

Call `super` correctly, so that we correctly initialise the `errcode` field.

Fixes https://github.com/matrix-org/synapse/issues/2179.

synapse/util/__init__.py

@@ -26,7 +26,7 @@ logger = logging.getLogger(__name__)
 
 class DeferredTimedOutError(SynapseError):
     def __init__(self):
-        super(SynapseError, self).__init__(504, "Timed out")
+        super(DeferredTimedOutError, self).__init__(504, "Timed out")
 
 
 def unwrapFirstError(failure):