onUnmapped

Replaces the problem produced for an exception no mapping covers.

The default is about:blank with no detail, carrying the status defaultExceptionStatusCode derives from the exception, not a flat 500. A problem document reaches the client (§5), so it must not leak a file path or SQL fragment, and Ktor's own NotFoundException must not become a 500 just because this library is installed.

This is the safe way to change catch-all behaviour. Unlike map<Throwable>, it only supplies the document, so the surrounding handler still rethrows cancellation and still picks log severity from the resulting status.