If the dialog controller has asked to show a dialog, then if we cannot show a
dialog, we should reject its request.

Not doing this results in the WebView becoming frozen for input if the load
fails, seemingly.

Change-Id: I7dddd5357c712f896499cdebb142e60b5e80b4f2
Reviewed-by: Pierre Rossi <pierre.rossi@theqtcompany.com>