Display i3-nagbar when commands lead to an error

e.g. pressing Mod1+x when having the following in your configfile:

    bindsym Mod1+x some invalid command

will lead to an i3-nagbar instance popping up, offering you to view the
error log (which will contain parser errors from this commit on).

generate-command-parser.pl

@@ -152,7 +152,7 @@ for my $state (@keys) {
         $cmd =~ s/[^(]+\(//;
         $cmd =~ s/\)$//;
         $cmd = ", $cmd" if length($cmd) > 0;
-        say $callfh qq|           printf("$fmt\\n"$cmd);|;
+        say $callfh qq|           fprintf(stderr, "$fmt\\n"$cmd);|;
         say $callfh '#endif';
         say $callfh "             state = $next_state;";
         say $callfh "             break;";