Fix parsing of #EXTMYTHTV tag in M3U playlists..

This caused any additional mythtv specific tags to be ignored
Do not display a popup that no channels were found when using IPTV.

IPTV doesn't use transport streams; so at the end of a scan we always got the message that no new channels were found, even when we did
Show the user an error if downloading an IPTV playlist failed

Any error would have silently discarded, only showing up in the log. So add a way to propagate the error and show a popup (to keep in the style of mythtv-setup)