pnfsmanager: Propagate cleaner errors to RemoteNameSpaceProvider

The NameSpaceProvider interface is not asynchronous and cleaning
a pool location through this interface should propagate errors
back to the client.

Target: trunk
Require-notes: no
Require-book: no
Acked-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>
Acked-by: Paul Millar <paul.millar@desy.de>
Patch: https://rb.dcache.org/r/7501/