i3bar: Amend status line error 127 message

Long description:

Exit 127 can be returned by the shell when the command is not found or
when the `status_command` process returns 127 because of a missing C
library dependency.

To apply this patch, use:
curl http://cr.i3wm.org/patch/371/raw.patch | git am

@@ -420,7 +420,7 @@ void child_sig_cb(struct ev_loop *loop, ev_child *watcher, int revents) {

     if (exit_status == 126)

         set_statusline_error("status_command is not executable (exit %d)", exit_status);

     else if (exit_status == 127)

-        set_statusline_error("status_command not found (exit %d)", exit_status);

+        set_statusline_error("status_command not found or is missing a library dependency (exit %d)", exit_status);

     else

         set_statusline_error("status_command process exited unexpectedly (exit %d)", exit_status);