Better docs, and possibly better handling, for 'packages' argument to javascript_catalog

[James Bennett]

The javascript_catalog i18n view's packages argument only accepts modules which are either django.conf or which are also in INSTALLED_APPS. But there are two issues with this:

1. The documentation for the view hides this a bit, and can create the impression that the restriction only applies to a packages argument passed through the URL.

2. The view itself does not give you any warning if you're violating this restriction; it just silently discards anything in packages that doesn't meet the requirements.

At the very least, the documentation should be more up-front that this requirement always applies no matter how you're passing packages to the view. Possibly as a bonus, the view should warn that it's discarding any packages that don't conform to the requirements, to aid in debugging.