Exceptions defined with `define-exception` don't receive a Signalable instance

[Jason94]

Types defined with define-exception don't receive a Signalable instance, but they probably should.

Reading the docs, it kind of seems like Coalton has two separate error mechanisms going on, Signalable / error and define-exception / throw? I'm not quite sure how they relate to each other, so maybe this is intentional?

---

With this exception type:

  (define-exception MyException
    (MyException String))

They can't be called with error:

(coalton (error (MyException "Error")))
===>
     1 |  (coalton (error (MyException "Error")))
       |           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expression has type ∀ :A. (SIGNALABLE MYEXCEPTION) => :A with unresolved constraint (SIGNALABLE MYEXCEPTION)
       |           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Add a type assertion with THE to resolve ambiguity

[stylewarning]

Related: Point #4 of https://github.com/coalton-lang/coalton/blob/main/docs/intro-to-coalton.md#caveats