add more informative error info for some common errors; also, no not exit with error when decoding to stdout and writing return EPIPE

make --serial-number or the default random serial number apply only to the first stream, and increment it for subsequent streams

print error if output file already exists, instead of overwriting by default; add -f (--force) option to force overwriting of files

when encoding to Ogg FLAC, set a random serial number (instead of 0) if one is not provided on the command-line