Issue Description:

First, here's a good error message:

func thrower() throws -> () {
    // doesn't matter what we put here
}
func f() -> () {
    try thrower()
}
// errors thrown from here are not handled

Yup, just about anyone can probably understand that. I wouldn't mind if it were more explicit; there are basically four solutions (`throws`, `do/catch`, `try?`, and `try!`) and a FixIt suggesting them would be great, but it's clear enough.

OK, now let's move the problem into a closure:

func doThis(f:()->()) {
    f()
}
func thrower() throws -> () {
    // doesn't matter what we put here
}
func f() {
    doThis {
        try thrower()
    }
}
// invalid conversion from throwing function of type '() throws -> ()' to non-throwing function type '() -> ()'

Experience (on Stack Overflow) shows that that just confuses people — a lot. I wonder if it might be made smart enough to say "errors thrown from here are not handled" instead? Thanks for considering this.