mount: improve error message for invalid filesystem names

For an invalid filesystem name used like this:
mount -t asdfs /dev/ada1p5 /usr/obj

emit an error message like this:
mount: /dev/ada1p5: Invalid fstype: Invalid argument

instead of:
mount: /dev/ada1p5: Operation not supported by device

Differential Revision: https://reviews.freebsd.org/D31540