Abort starting / continuing a game upon any kind of error from loading JSON game data.

[BevapDin]

This kind of relates to #18130. If the game encounters an error while loading the JSON data, it cancels loading the game data (items etc.), but otherwise continues loading / starting the actual game. This leads to a various errors about missing definitions.

The PR changes the logic to abort starting / continuing a game and going back to the main menu.

The "load" functions in class game now throw an exception (or simply propagate the exception throw from within further function calls). Callers catch those exceptions:

• dump_stats reports them via std::cerr (as the curses system is not initialized there, see surrounding code) and returns immediately,
• same for game::check_mod_data,
• same for code in tests/.
• game::quickload reports via debugmsg, technically the program should just exit as the game may be in any invalid state, but the user could try to fix the problem and do another quickload.
• Code in main_menu.cpp will just abort loading/starting a game. This should be safe, users can start a new game / load another game without leaving the program.

To test this, add some errors to the JSON files and try to start/load a game. It should print a debug message and put you back into the menu.

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This is technically an alternative to #18136, it fixes the "When you try to play the game, it throws what must be hundreds of errors at you, saying it can't find tons and tons of stuff" issue.

But #18136 is not an alternative to this PR. #18136 won't fix the main problem: the program allows you to start a game with faulty JSON data. #18136 allows one to reach the actual game despite redundant JSON values.