Friendlier error message when server does not support UDP port range.

scripts/mosh

@@ -137,7 +137,7 @@ if ( defined $port_request ) {
       }
     }
   } else {
-    die "$0: Server-side port ($port_request) not valid.\n";
+    die "$0: Server-side port or range ($port_request) is not valid.\n";
   }
 }
 
@@ -245,8 +245,9 @@ if ( $pid == 0 ) { # child
   my $quoted_self = shell_quote( $0 );
   exec "$ssh " . shell_quote( '-S', 'none', '-o', "ProxyCommand=$quoted_self --fake-proxy -- %h %p", '-t', $userhost, '--', "$server " . shell_quote( @server ) );
   die "Cannot exec ssh: $!\n";
-} else { # server
+} else { # parent
   my ( $ip, $port, $key );
+  my $bad_udp_port_warning = 0;
   close $pty;
   LINE: while ( <$pty_slave> ) {
     chomp;
@@ -262,6 +263,9 @@ if ( $pid == 0 ) { # child
 	die "Bad MOSH CONNECT string: $_\n";
       }
     } else {
+      if ( $port_request =~ m{:} and m{Bad UDP port} ) {
+	$bad_udp_port_warning = 1;
+      }
       print "$_\n";
     }
   }
@@ -272,6 +276,9 @@ if ( $pid == 0 ) { # child
   }
 
   if ( not defined $key or not defined $port ) {
+    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";
   }