audio: avoid duplicated error messages on init failure
dec_audio.c init_audio_codec() would in one case print
"ADecoder init failed :(\n" and return failure. Its only caller
init_best_audio_codec() printed exactly the same message if the
returned result was failure. Change the latter message to say
"Could not open audio decoder %s.\n" instead. Some of the
per-open-attempt messages are kind of value about their context; this
new message should make it more clear where the attempt to open one
specific codec ends.