nspawn: add a hint to the message we emit when a child dies

From #10526:

$ sudo systemd-nspawn -i image
Spawning container image on /home/zbyszek/src/mkosi/image.
Press ^] three times within 1s to kill container.
Short read while reading cgroup mode.

src/nspawn/nspawn.c

@@ -3862,7 +3862,8 @@ static int run(int master,
                 if (l < 0)
                         return log_error_errno(errno, "Failed to read cgroup mode: %m");
                 if (l != sizeof(arg_unified_cgroup_hierarchy)) {
-                        log_error("Short read while reading cgroup mode.");
+                        log_error("Short read while reading cgroup mode (%zu bytes).%s",
+                                  l, l == 0 ? " The child is most likely dead." : "");
                         return -EIO;
                 }
         }