Improve Result and exception handling ergonomics

[mayoff (Rob Mayoff)]

@pronebird

Been waiting for some sort typed throws ErrorType for years but not seeing it anywhere on the horizon

John McCall referred to this as “precise error typing” in September 2021.

Precise error typing in Swift

Introduction When error handling was added to Swift in 2015, it intentionally did not allow the expression of precise error types: either a function cannot throw, or it can throw any Error. The error-handling rationale I wrote, which uses an awful lot of words to describe most aspects of the design, gives this one pretty short shrift: There are many kinds of error. It's important to be able to recognize and respond to specific error causes programmatically. Swift should support easy pattern-m…

I have no idea if there's been any compiler work on it, but it was (afaik) the first time McCall endorsed the idea.