pool: Improve resilience against broken SI files

Addresses an issue in which corrupt SI files could cause pool
startup to fail. Eg. NPE or OOM during deserialization would
cause the pool to fail and it would not even log the file
triggering the problem. Such exceptions would typically be
caused by corrupt length fields or similar invalid data in
the SI file.

Also improved the exception chaining and logging in this part
of the pool.

The problem was reported on the user-forum list.

Target: trunk
Request: 2.6
Request: 2.2
Acked-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>
Patch: http://rb.dcache.org/r/5609/