Create a TranscoderOptionsError class and ui that inherits from
TranscoderOptionsInterface. Use this to display options errors. Move
widget creation into a static method. These changes will allow use of
the same mechanism in the transcoder settings page.
Remove condition that allows no dialog to be displayed when user selects
options for transcoding. Display the dialog with an error message
instead. This also applies to the transcode options dialog in the
ripping and network remote settings.