Check if the Resources are empty

[torkleyy]

If a resource couldn't be fetched and Resources is empty, it might be because somebody accidentally created a new instance by specifying it as SystemData. This should be pointed out in the panic.

[torkleyy]

cc @Moxinilian

[cedric-h]

Would it also be reasonable to add some way to iterate over the types of resources stored? I recently found that I needed this, and somebody else on the specs github issues found that they needed this as well. (amethyst/specs#559)
Since it's stored in a hash map, that could actually be as simple as making the resources field public, although I'd prefer some other sort of API. If this is desirable, what would you recommend? I'm willing to write the PR.

[cedric-h]

I could also do the variable renaming part of #111 while I'm over there.
Well I guess that should be a different PR. Nevermind.

[torkleyy]

Would it also be reasonable to add some way to iterate over the types of resources stored?

Yes, I think that would be useful, too. I think the simplest solution would be to just create a method that returns an iterator over the values, yielding &Resource. The keys aren't really interesting since they're just the type ids.

[torkleyy]

@DefecateRainbows are you still interested in this?