btrfs: Proper error when output of 'btrfs qgroup show' command cannot be parsed.

sscanf doesn't set errno in these cases so it's wrong to call perror
here.

This fixes commit 2b7f7810ed.

daemon/btrfs.c

@@ -1275,7 +1275,7 @@ do_btrfs_qgroup_show (const char *path)
     if (sscanf (line, "%" SCNu64 "/%" SCNu64 " %" SCNu64 " %" SCNu64,
                 &dummy1, &dummy2, &this->btrfsqgroup_rfer,
                 &this->btrfsqgroup_excl) != 4) {
-      reply_with_perror ("sscanf");
+      reply_with_error ("cannot parse output of qgroup show command: %s", line);
       goto error;
     }
     p = strchr(line, ' ');