Unclear error message when running btrfs check on a mountpoint

Hi!

With kernel 4.3-rc7 and btrfs-progs 4.2.2 I get:

merkaba:~> btrfs check /daten
Superblock bytenr is larger than device size
Couldn't open file system

It took me a moment to see that I used a mountpoint and that this may be the reason for the error message.

Maybe check for a device file as argument and give a clearer error message in this case?

Thanks,

Martin