Nicer error messages on a likely forgotten resume

Students (and many other users like me) often forget to resume from a handler. We could customise the error message, especially when it's obvious from the type that using resume would fix the type error one gets.

Let's take a look at a small example:

effect get(): Int

def foo() = 
  try { println(do get()) }
  with get {
    42
//  ^^
//  Expected Unit but got Int.
  }

We could customise ( 🚲🏠 ) the error to something slightly more specific like:

Expected Unit but got Int.
Did you mean to use `resume: Int => Unit` in the handler in order to resume?

The heuristic is pretty straightforward: an additional hint should fire if in the handler we get "expected τ1 but got τ2" and resume has type τ2 => τ1.

effekt-lang / effekt

Nicer error messages on a likely forgotten resume #680