psycopg: Don't include conn info in exception

When trying to get a pooled_psycopg2 connection, the _get_pool() method
checks the connection args against the pool args and raises an exception
if the two differ. This leads to an internal server error, which results
in logging the connection and pool info / args, which include the DB
password, and, depending on the deployment, it might also send them via
email to the admins. Log the connection and pool args only when log
level is set to  debug, and remove them from the exception message, so
that they won't end up being send via email.