[web] Return useful error messages when exceptions arise in ajax controllers (closes #3932503)
The call to super() in RemoteCallException passes 'reason' all the way to
python's Exception.__init__() which is then stored in Exception.args. This way,
CubicWebPublisher.ajax_error_handler() gets a useful error message when calling
unicode(exc).
The change to uilib is just to prepend the exception type name. Just a small
improvement.