Beginnings of the Failure exception.

The handling of fail operations has changed, a Failure exception
is now thrown if the operation's response has a client or server
error. The exception might be rescued by the operation dispatcher
in the following cases:

- The request is an XHR: the response of the operation is
returned.

- The operation was "forwarded" (see below): the exception is
discarted and the dispatch of the request continue.

An operation is considered "forwarded" when the actual destination
and operation type is defined using the request parameters
DESTINATION and NAME. The URI of the request is irrelevant to
forwarded operation, and if they fail the dispatch process simply
continue. This allow forms to be posted to their own URL (not
the URL of the operation) and displayed again if an error occurs.
Updated with 'Failure' and 'forwarded'
Typo
Fix: The response of successful forwarded operation is not discarted.
Fix: Forwarded successful operation responses are discarted even when the request is an XHR.