pool: Fix race condition that leads to orphaned files

Using FileNotInCacheException cause the chimera registration to be cleared.
Clearly we don't want this when the file is actually in the pool and just
happens to be in a state in which it isn't accessible.

This patch changes the exception to LockedCacheException.

This is an alternative to patch: https://rb.dcache.org/r/5801

Target: trunk
Request: 2.9
Request: 2.8
Request: 2.7
Request: 2.6
Request: 2.2
Require-notes: yes
Require-book: no
Acked-by: Paul Millar <paul.millar@desy.de>
Patch: https://rb.dcache.org/r/7103/