Improve error message for optional arguments mismatches

[dbuenzli]

I spent little but somehow too much time alpha renaming these expressions to see where the problem could be before realizing it was simply an optional vs non optional issue (maybe I'm tired).

Could maybe the error message be improved to directly mention optionability differences ?

       Values do not match:
         val v3 :
           ?fold:('a, 'b) Vec.fold ->
           ?make:('b -> 'b -> 'b -> 'a) -> 'b t -> 'a t
       is not included in
         val v3 :
           fold:('v3, 'a) Vec.fold ->
           make:('a -> 'a -> 'a -> 'v3) -> 'a t -> 'v3 t
       The type
         ?fold:('a, 'b) Vec.fold ->
         ?make:('b -> 'b -> 'b -> 'a) -> 'b t -> 'a t
       is not compatible with the type
         fold:('c, 'd) Vec.fold ->
         make:('d -> 'd -> 'd -> 'c) -> 'd t -> 'c t

(OCaml v4.14.1)