pool: Log P2P fatal errors with stack trace

We have observed class initialization errors at NDGF, but the code
doesn't log a stack trace for subclasses of Error. This patch solves
that problem.

Target: trunk
Request: 2.9
Request: 2.8
Request: 2.7
Request: 2.6
Require-notes: yes
Require-book: no
Acked-by: Paul Millar <paul.millar@desy.de>
Patch: http://rb.dcache.org/r/6988/