fix pb with bytes field processing: currently when an existing file is edited and no new file is specified, it finds no value in req.form and so try to detach the current file, which is wrong. In that case, nothing should be done. So introduce a new UnmodifiedField exception that may be raised in field's process_form_value method (catched in field.process_posted).

"""Common subpackage of cubicweb : defines library functions used both on the
hg stserver side and on the client side

:organization: Logilab
:copyright: 2001-2009 LOGILAB S.A. (Paris, FRANCE), license is LGPL v2.
:contact: http://www.logilab.fr/ -- mailto:contact@logilab.fr
:license: GNU Lesser General Public License, v2.1 - http://www.gnu.org/licenses
"""