ftp: Log stack trace on authentication failure

A recent patch removed logging of stack trace during GSI/Kerberos
authentication failure. Although it is correct not to log a stack
trace for such a failure (it is not a bug), having a stack trace
while debugging JGlobus 2 is convenient. This patch adds logging
of the stack trace at trace level.

Target: trunk
Request: 2.6
Require-notes: no
Require-book: no
Acked-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>
Patch: http://rb.dcache.org/r/5547/