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/commands.c

@@ -1443,6 +1443,7 @@ void cmd_exit(I3_CMD) {
 void cmd_reload(I3_CMD) {
     LOG("reloading\n");
     kill_configerror_nagbar(false);
+    kill_commanderror_nagbar(false);
     load_configuration(conn, NULL, true);
     x_set_i3_atoms();
     /* Send an IPC event just in case the ws names have changed */