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).

src/util.c

@@ -303,6 +303,7 @@ void i3_restart(bool forget_layout) {
     char *restart_filename = forget_layout ? NULL : store_restart_layout();
 
     kill_configerror_nagbar(true);
+    kill_commanderror_nagbar(true);
 
     restore_geometry();