In the "Advanced" section there is field "Regular expression for failed builds".
The help [(?)-button] shows:
"If set, this regular expression will be used instead of the regular regular expression when the build has failed."
I read this as: "If the field is empty, the RE from the field "Regular expression:" is also used for the failed builds.

This didn't work for me. I had to add the same RE to the "Regular expression for failed builds" to get a description set for the failed builds.