btrfs-progs: Improve the errno string about open_path_or_dev_mnt()

btrfs-progs: Improve the errno string about open_path_or_dev_mnt()

open_path_or_dev_mnt() is used to on *mounted* btrfs device or mount
point, when a unmounted btrfs device is passed, errno is set to EINVAL to
info the caller.
If ignore the errno and just print "ERROR: can't access '%s'", end users
will get confused.

This patch will add check for open_path_or_dev_mnt() caller and print
more meaningful error message when a unmounted btrfs device path is
given.

Signed-off-by: Qu Wenruo <quwenruo@cn.fujitsu.com>
Signed-off-by: David Sterba <dsterba@suse.cz>