[FIX] http: force rolleback for failed http request

1st issue:
When an exception was raised, it was badly handled by the website in case of
website_enabled key. The response page was generated without calling super.
The WebRequest object being responsible to rollback the transaction in case
of errors.

2sd issue:
The _failed attribute is required to rollback the transaction in an WebRequest
object. Previously it was only set in the JsonRequest object (which inherit
from WebRequest), replace by call to super. The attribute _failed is now set
in the WebRequest object.