This view will be selected upon any exception which is a subclass of
cubicweb.web.RequestError when the request has Accept:application/json.
Previously, the "generic_error" view would have been called, but it
cannot accurately handle CubicWeb's web exceptions (it always returns
500, whereas these web exceptions may have a different status code).

Why are these views selected? This is because the RootResource matches
for any request with this Accept:application/json header. (In fact, we'd
want to use this error view only when the request comes from a
sub-resource of the RootResource; but that's not easy to achieve.)

So this new error view convert a cubicweb.web.RequestError into an
application/problem+json response. This is thus not backward compatible
with what CubicWeb would return.

In added test, we use the "format_date" "ajax" controller (of CubicWeb)
with missing argument which then responds with a RemoteCallFailed (with
a 500 status) that is called by our view.