Error message
In the message, %s marks a value that the engine fills in when it raises the error.
The story
You asked ls to list a directory, and it could not read it. Ls works on a real folder on your machine. When the operating system refuses, rush passes along both pieces of information: the directory exactly as you wrote it, and the system's own reason. There are three usual reasons. The directory might not exist, often because of a typo or because you are in a different place than you thought. It might not be a directory at all, but a plain file. Or you might not have permission to look inside. The old message just said the builtin refused, which told you nothing. Now you can act immediately. Picture asking a receptionist for the list of people on the third floor, and hearing the building has no third floor, versus hearing no. The first answer helps. So read the reason in the message. Fix the path, or the permissions, and ls will list the folder as expected. Small message, big time saver.
Text from the Rush engine's error catalogue.