Add a little hint for first-time users

I was recommended to use mosh as "an in-place replacement for ssh" by
my friends, and take a while to figure out the message until I
realized I missed the line "The Mosh package should be installed on
both the client and server."

If this line often corresponds to missing server-side installation,
rather than other mysterious scenario, how about adding a hint?

scripts/mosh.pl

@@ -445,7 +445,7 @@ if ( $pid == 0 ) { # child
     if ( $bad_udp_port_warning ) {
       die "$0: Server does not support UDP port range option.\n";
     }
-    die "$0: Did not find mosh server startup message.\n";
+    die "$0: Did not find mosh server startup message. (Have you installed mosh on your server?)\n";
   }
 
   # Now start real mosh client