srm/transfermanager: make errors less noisy

If a third-party transfer fails, it gets logged in the pool,
transfermanager general log, transfermanager activity log and
the SRM.  The log messages are also needlessly long and include
unnecessary information.

This patch quietens this considerably.

Target: master
Patch: http://rb.dcache.org/r/6886/
Acked-by: Tigran Mkrtchyan