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.

[Aymeric Augustin]

Two problems are reported in this ticket:

(a) the ​documentation of javascript_catalog is confusing

(b) invalid package names are silently dropped: yes, silent failure is bad and we should do something. Raising an exception would be backwards incompatible. Raising a warning could be a good compromise.

I've created #18596 to track (a). Let's focus this ticket on (b).

[Tim Graham]

Claude, I know made some improvements when you created the new JavaScriptCatalog class-based view in 1.10. Is this ticket still relevant for that view?