[FIX] core: log import traceback in debug mode

This is a followup of 1d553eea.

Systematically logging import tracebacks can fill up log files, and
waste disk space.  We therefore log those tracebacks in DEBUG mode, so
that they can be activated only when needed.

closes odoo/odoo#44937

Signed-off-by: Raphael Collet (rco) <rco@openerp.com>