srm: Add SRM status code to SRMException

Makes it easier to translate exceptions to an SRM reply. Unused
in this patch, but is used in subsequent patches.

Target: trunk
Require-notes: no
Require-book: no
Acked-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>
Patch: http://rb.dcache.org/r/6659/