namespace: Improve error message in upload cancellation

The error message was a bit misleading.

Target: trunk
Require-notes: no
Require-book: no
Acked-by: Paul Millar <paul.millar@desy.de>
Acked-by: Albert Rossi <arossi@fnal.gov>
Patch: http://rb.dcache.org/r/7007/