Error message
In the message, %s marks a value that the engine fills in when it raises the error.
The story
You waited for an answer, your timeout ran out, and rush said the outcome is unknown. That sentence is honest, and here is why. When you ask rush to write something, your request goes into a queue. At some point the one reader and writer takes it. After that moment, the request is committed and cannot be called back. Now suppose the database is slow, and your timeout passes first. Did the write land? Maybe yes, maybe no. A database program that was stopped right after receiving the commit might even have finished it. Saying cancelled would be a lie, because it may well have happened. So rush says unknown, and it replaces the stuck lane, so your next request does not wait behind it. But if your request was still waiting in the queue when time ran out, it was never taken, so it is simply withdrawn, and nothing happened at all. The second slot tells you which case you are in. If the outcome is unknown, check the holder before you retry, so you do not write the same thing twice.
Text from the Rush engine's error catalogue.