cells: Remove "Sending CellException to" log message

Addresses an issue of frequent "Sending CellException to" messages in log files.
This is a regression introduced when we removed the last pieces of the old
logging system from dCache. These messages used to be surpressed, but now
happen to be logged.

The patch improves the text of the log message and changes the log level to
debug.

Target: trunk
Request: 2.6
Require-notes: yes
Require-book: no
Acked-by: Albert Rossi <arossi@fnal.gov>
Patch: http://rb.dcache.org/r/5665/