Expose handler for message bus errors

[jdm]

Per this IRC conversation, it would be easier to diagnose system configuration issues if the webrtc code did not silently swallow message bus error messages.

[jdm]

The audio decoder does this; not clear if any other code handles bus error messages.

[philn]

for the MediaPlayer, as it's based on GstPlayer (AFAIK), you could rely on the error signal, https://slomo.pages.freedesktop.org/rustdocs/gstreamer/gstreamer_player/struct.Player.html#method.connect_error

[philn]

Ah that's done in the player, already, https://github.com/servo/media/blob/master/backends/gstreamer/src/player.rs#L565