btrfs-progs: add the error message when open fails

When open in btrfs_open_devices failed, only the following message is
displayed. Therefore the user doesn't understand the reason why open
failed.

  # btrfs check /dev/sdb8
  Couldn't open file system

This patch adds the error message when open fails.

Signed-off-by: Tsutomu Itoh <t-itoh@jp.fujitsu.com>
Signed-off-by: David Sterba <dsterba@suse.com>

volumes.c

@@ -227,6 +227,8 @@ int btrfs_open_devices(struct btrfs_fs_devices *fs_devices, int flags)
 		fd = open(device->name, flags);
 		if (fd < 0) {
 			ret = -errno;
+			error("cannot open device '%s': %s", device->name,
+					strerror(errno));
 			goto fail;
 		}