Let nightly.getAppIniString fail gracefully. Workaround for #125.

Workaround for #125.

These commits make Copy/Insert Build ID menuitem to produce IDs like the following:
> Mozilla/5.0 (Windows NT 6.1; WOW64; rv:17.0) Gecko/17.0 Firefox/17.0 ID:20121119183901 CSet: Undefined