srm: Make rmdir work with S2

Addresses an issue with extracting the path from S2 URLs.

S2 uses SURLs were the path in specified as a query parameter.
Recent code added to srmRmdir processing failed to extract
such paths.

Also fixed a bug in CacheExceptionFactory that caused the
rmdir implementation to return the wrong error message
when attempting to remove non-directory entries.

Target: trunk
Require-notes: no
Require-book: no
Acked-by: Paul Millar <paul.millar@desy.de>
Patch: http://rb.dcache.org/r/6159/