Core: clarify error message when reading an APContainer

[beauxq]

What is this fixing or adding?

Since any game can override read_contents in APContainer (and might need to),
we could use a more clear error message if something goes wrong with that implementation.

A suggestion was made to do something with the version number that's defined in core.
But if the plan is to move games out of core, or have better support for games outside of core, then we can't use a patch version number in core to keep track of changes in game implementations.

The solution needs to be something that is available to games that have never been in core and never will be in core.

How was this tested?

I created a patch before this change: #2875
Then tested opening that patch with #2875 with and without these changes.

Before:

KeyError: "There is no item named 'gen_data.json' in the archive"

After:

KeyError: "There is no item named 'gen_data.json' in the archive"

The above exception was the direct cause of the following exception:

...

worlds.Files.InvalidDataError: There is no item named 'gen_data.json' in
the archive - This might be the incorrect world version for this file

[black-sliver]

does len really make sense here over just if e.args?

[beauxq]

I dislike the python-ism of empty: false, non-empty:true
I've seen other people (in AP discord) say they dislike it also.
It doesn't match other languages, and can take people by surprise. So I prefer to avoid using it.

[black-sliver]

This change is completely separate from why the version number exists and needs to be bumper. The version number is to make sure that the Patch program can understand what the patch is, not that the Patch program can actually patch the file.

This change for better error reporting is useful regardless of that.