I really like your explanation here. It clearly shows that there definitely are important scenarios where having a feature like you pitched here would be really nice.
However, as others pointed out already, your pitch seems to actually just be a part of the much larger feature typed throws that was already heavily discussed a while ago. Even the core team (or at least members thereof) showed some interest at some point. However, the discussion somehow has come to nothing and slowly died off. Maybe you could revive it by providing your excellent explanation there. Because your use-case would definitely be covered by the larger feature and this solution would fit the language much better, IMHO, since it doesn't introduce a little feature that goes against anything related in Swift, but rather really fixes the underlying error.
EDIT:
Just scrolled to the bottom of that thread and saw that you already engaged in the discussion there with a similar argument. Nevermind then