[FIX] bus: prevent error popups when longpolling requests are interrupted

1b9c404 modified the /longpolling/poll requests
to use openerp.session.rpc() instead of openerp.jsonRpc().
In order to properly disable error popups, an
explicit preventDefault() call is now required.
Merge branch '8.0' of https://github.com/odoo/odoo into 8.0-linux64