To Called within an except block to allow converting exceptions to arbitrary responses. Anything returned (except None) will be used as response. use this code given below in openerp.

def _handle_exception(self, exception):
        """Called within an except block to allow converting exceptions
           to arbitrary responses. Anything returned (except None) will
           be used as response."""
        try:
            return super(JsonRequest, self)._handle_exception(exception)
        except Exception:
            if not isinstance(exception, (openerp.exceptions.Warning, SessionExpiredException, openerp.exceptions.except_orm)):
                _logger.exception("Exception during JSON request handling.")
            error = {
                    'code': 200,
                    'message': "Odoo Server Error",
                    'data': serialize_exception(exception)
            }
            if isinstance(exception, AuthenticationError):
                error['code'] = 100
                error['message'] = "Odoo Session Invalid"
            if isinstance(exception, SessionExpiredException):
                error['code'] = 100
                error['message'] = "Odoo Session Expired"
            return self._json_response(error=error)