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)

[Octachron]

With #13260 merged, the error message now points out the mismatch on the label

       Values do not match:
         val x :
           ?fold:('a, 'b) Vec.fold ->
           ?make:('b -> 'b -> 'b -> 'a) -> 'b t -> 'a t
       is not included in
         val x :
           fold:('a, 'b) Vec.fold ->
           ?make:('b -> 'b -> 'b -> 'a) -> 'b t -> 'a t
        ...
       The label ?fold was expected to not be optional