fix display when proxied GET yields ERROR

The error message is not displayed to the use, but this mirrors the
behavior when a regular get from a special remote fails. At least now
there is not a protocol error.

P2P/Protocol.hs

@@ -623,6 +623,8 @@ receiveContent mm p sizer storer mkmsg = do
 				validitycheck
 			sendSuccess (observeBool v)
 			return v
+		Just (ERROR _err) ->
+			return observeFailure
 		_ -> do
 			net $ sendMessage (ERROR "expected DATA")
 			return observeFailure