srm: Improve error and status messages

Addresses issues in error and status messages. End-users
should not be confronted with exception names and other
internal and code specific messages.

Target: trunk
Require-notes: no
Require-book: no
Acked-by: Paul Millar <paul.millar@desy.de>
Acked-by: Dmitry Litvintsev <litvinse@fnal.gov>
Patch: http://rb.dcache.org/r/6160/