Backport #1709

[ncik-roberts]

Backport ocaml/merlin#1709 to fix misleading type errors like this:

type t = {
    foo : int
    bar : baz
}

(*
A type variable is unbound in this type declaration.
In field foo: 'a the variable 'a is unbound
*)

@liam923 please merge once you review this.

[liam923]

LGTM, but I'll wait until #58 merges to merge because if there's conflicts, they'll be easier to resolve in this PR.