$ ffmpeg -v error -i 123.mp4 -vf "select='eq(n,0)', alphaextract" -f null /dev/null

[Parsed_alphaextract_1 @ 0x7fe8f5208100] Requested planes not available.
[Parsed_alphaextract_1 @ 0x7fe8f5208100] Failed to configure input pad on Parsed_alphaextract_1
[vf#0:0 @ 0x7fe8f5005f40] Error reinitializing filters!
Failed to inject frame into filter network: Invalid argument
Error while filtering: Invalid argument
[out#0/null @ 0x7fe8f5004900] Nothing was written into output file, because at least one of its streams received no packets.

如果不包含透明通道会打印错误信息，且程序退出码为非0。