Error message
In the message, %s marks a value that the engine fills in when it raises the error.
The story
Something inside an expression of yours failed, and this message points you at which one. Here is how it works. When you use map, filter or reduce, rush walks your element expression once for every member of the collection. When you use process.await or process.done, rush walks the read you are waiting on, fresh each time it checks. If the expression itself refuses, say it divides by zero or reads something missing, that refusal is the real reason, and normally you see it directly. This particular message appears when the inner reason did not travel back up to the builtin. Instead of leaving you with a blank failure, rush still tells you which expression was the one that stopped, by naming its argument in the first slot. Picture a map over a list of prices, where one entry is broken. You now know the map expression is where to look. Start there, test the expression on a single member, and you will usually find the culprit quickly. The name is the clue, so follow it.
Text from the Rush engine's error catalogue.