This event is emitted when an AJAX request encounters fatal error like a timeout or loss of network connectivity.
Note that this event will not be emitted when the server produces an
error message with an HTTP status like
500. When the server can produce
up:request:loaded is emitted instead.
The event is emitted on the layer that caused the request.
The failed request.
The layer this request is associated with.
If this request was intended to update an existing fragment, this is that fragment's layer.
If this request was intended to open an overlay, the associated layer is the future overlay's parent layer.
The link or form element that caused the request.