Just flushing the bus doesn't work here for some reason, so set
playbin to NULL state, which seems to clear all error state and
makes sure we do play the next playable song and don't pick up
'ghost' error messages from previous files on the bus.
Differs from a plain gst-launch-1.0 playbin uri=... pipeline in that
it can take multiple arguments and as such allows testing of things
like gapless playback, switching between different formats and the
like. Very minimal at this point, we'll probably want to add
interactive controls and more options at some point.
https://bugzilla.gnome.org/show_bug.cgi?id=553520
