audio: improve decoder open failure handling
Fix handling of unsupported lavc output format to prevent an endless
stream of errors on init. Print a more informative error message if
the problem is due to lack of libavresample support. Reinitialize
sh_audio->samplesize and sample_format before falling back to another
audio decoder (some decoders rely on default values). Remove code
setting these fields from demux_mkv and demux_lavf (no decoder should
depend on demuxer-set values for these fields).