FFmpeg Media Plugin: Print the error description of the ffmpeg framework, to provide more clarity to the error reason.

Signed-off-by: Colin Günther <coling@gmx.de>
FFMPEG Media Addon: Implemented audio streaming support for the decoder part. This definitely needs some refactoring, as the current implementation is hard to grasp and has potentially introduced new bugs (at least my gut is telling me that).

Signed-off-by: Colin Günther <coling@gmx.de>
FFMPEG Media Addon: Finetune mp3 decoding test, so that it cleans up nicely when done playing.

Signed-off-by: Colin Günther <coling@gmx.de>