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).

include/all.h

@@ -54,6 +54,7 @@
 #include "i3.h"
 #include "x.h"
 #include "click.h"
+#include "key_press.h"
 #include "floating.h"
 #include "config.h"
 #include "handlers.h"