Improve error message when let! _ is last statement in computation expressions

Consider the following code:

let x() = async { return () }

let y =
async {
let! _ = x()
} // compile error - Unexpected symbol }

let z =
async {
do! x()
}

If `x` returns anything else that unit then you can't use `do!` and you have to do:

let y =
async {
let! _ = x()
()
}

This is hard to explain to newcomers - especially given the error mesage.