This PR restores a specialized error path for mixed record labels in record patterns:
on the expression side, mixing record labels (from different record types) inside a record expression

type t = {x:unit}
type s = {y:unit}
let r = {x=();y=()};;

yields a specialized error message:

Error: The record field y belongs to the type s
but is mixed here with fields of type t

Contrarily, the error message for erroneous record patterns

let f = function {x;y} -> x

is the generic one

Error: This pattern matches values of type s
but a pattern was expected which matches values of type t

This difference is due to a small exception imbroglio: the label typing code was expecting an Unify exception after a call to unify_pat_types but unify_pat_types catches Unify exception by itself and transform them into Pattern_type_clash errors.