better error postcard API

### I am planning on implementing this myself

Yes

### Describe your request

currently, custom error postcards are implemented by switching to `LevelEnter` with an error message.
this is not an ideal solution, as the current scene still has to finish what it's doing, which could be in an invalid state.

the proposed better API would look like
```cs
throw new PostcardException("{big}Oops!{/big}{n}You did something bad!");
```
and would immediately stop processing the current scene and switch to a postcard.

### Additional context

_No response_