srm: fix IllegalStateTransition for COPY requests

If the user cancels a COPY request then the file request is moved to
state CANCELLED; however, if the copy then succeeds anyway, the SRM
attempts to move the job to state DONE.  This cannot succeed since
CANCELLED is a final state.

Fix this by first checking whether the job is already in a final
state.  Also tidy up some messages.

Target: master
Request: 2.10
Requires-notes: no
Requires-book: no
Acked-by: Tigran Mkrtchyan
Patch: https://rb.dcache.org/r/7147
Merge pull request #702 from paulmillar/fix/2.10/srm-illegal-state-transition

srm: fix IllegalStateTransition for COPY requests