btrfs-progs: refine btrfs-debug-tree error prompt when a mount point given

btrfs-progs: refine btrfs-debug-tree error prompt when a mount point given

Now, if exec:
	# btrfs-debug-tree <mount_point>
it echos:
	: Superblock bytenr is larger than device size

But it is quite misleading, because it is a valid btrfs.
In this case, we should tell the developer to provide a block device.

After apply:
	: '<mount_point>' is not a block device

Signed-off-by: Gui Hecheng <guihc.fnst@cn.fujitsu.com>
Reviewed-by: Satoru Takeuchi <takeuchi_satoru@jp.fujitsu.com>
Signed-off-by: David Sterba <dsterba@suse.cz>