refactor form error handling: * almost everything is in the base form class, with other form errors related code * deprecate form_field_error in favor of field_error and format_error on the renderer * fix pb with js localization of errors by properly using field.role_name() * rename 'errors' into 'error' in forminfo to avoid confusion

# cube's specific schema
"""

: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
"""