Expanded error message for universal quantification failure #1993
Currently, type expressions with an incorrect universal quantification, for instance
type a = <m: 'a. int as 'a >
which seems to imply that the universal variable is somehow not a variable.
This PR proposes to remove the ambiguity on the
which is quite similar to the related error message
type b = <m: 'a 'b. 'a as 'b >