Browsers don't have the functionality to fully support the generic
SDL_ShowMessageBox(), but this handles the likely most-common case.
Without this, you'd return immediately with a proper error result and no UI,
but probably no one checks that for SDL_ShowSimpleMessageBox. And if they
did: what would they do to handle this anyhow?
We'd need to lobby for an HTML spec of some sort that allows customizable
message boxes--that block!--to properly support SDL message boxes on
Emscripten, but this is probably Good Enough for now.
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