srm: Add error message to an IllegalArgumentException

After upgrading to 2.9, we suffered from a hard to find IllegalStateException.
This patch adds an error message to the exception.

Target: trunk
Require-notes: no
Require-book: no
Acked-by: Paul Millar <paul.millar@desy.de>
Patch: http://rb.dcache.org/r/6803/