What would you rather it print?

1) I don't know how to mount that.  (Isn't that what it just said?)

2) M781Z892Y892 - Invalid DOS partition table.  (How is it supposed to
do that?  Extend the system call semantics to return both an exact error
code/message and an exact error code?  How useful is this?  Does it
justify the cost?  Does it justify creating a new interface?)

3) mount_msdos: /dev/zd0a: Invalid argument.  This may not be a valid
file system.  You might have type the device name wrong, or you might
have put the wrong disk in, or you might have screwed up the partition
table, or you might have lost your mind and tried it on the wrong
machine, or ...  (Or maybe you ought to RTFM?)

I'm a bit skeptical.