pool: Improve logging of common errors

Improve log levels and log messages of common errors found in
production log files.

Target: trunk
Require-book: no
Require-notes: yes
Ticket: http://rt.dcache.org/Ticket/Display.html?id=7685
Acked-by: Paul Millar <paul.millar@desy.de>
Acked-by: Dmitry Litvinsev <litvinse@fnal.gov>
Patch: http://rb.dcache.org/r/5255/
(cherry picked from commit 5302c40b0942d7d09f5b8820a62f162d827e6957)
Merge pull request #74 from gbehrmann/fix/2.6/rb5255

pool: Improve logging of common errors