fix(SelectionScreen): be more robust in the face of save files with incomplete manifests

[keturn]

Fixes #4912

Notes for Reviewers

The mainWorldDisplayName method I factored out to GameManifest is very much a "view" type of method, given the way it returns error strings. I'd accept suggestions of other places to put it. Especially if that place had dependency injection support so the caller doesn't have to explicitly pass WorldGeneratorManager.

How to test

Edit the manifest.json of one of your save files so the worlds entry is the empty set: "worlds": {},

Start Terasology and go to the game loading screen, make sure that doesn't crash.
Hit the "Details" button, make sure that doesn't crash.

If you proceed to try to actually load the game, that will crash (we can only do so much with a broken save!). Hopefully with a relevant error message.

[keturn]

I broke it? I thought I had style checks turned on locally. Guess I missed something!

[jdrueckert]

Removing the worlds from a save's manifest.json successfully reproduced the crash on current develop.
With this PR checked out, the game does neither crash when opening the selection screen nor when opening the save details.
On loading the game it crashes as expected with the error message "Game manifest does not contain a MAIN_WORLD" which I think describes the problem pretty well 👍

[jdrueckert]

From the top of my head, I don't have a good idea for a suitable alternative place for mainWorldDisplayName. If you want to wait for other reviewers' ideas, feel free to do so.

If not, from my side this is good to be merged as it fixes the underlying issue 👍