improve warning message slightly

Author: AlexKnauth

typed-racket-lib/typed-racket/private/type-contract.rkt

@@ -547,7 +547,12 @@
        [(Opaque: p?)
         (when exact?
           (eprintf
-           "warning: Opaque predicates cannot be used as exact decisions yes/no:\n  predicate: ~s\n"
+           (string-append
+            "warning: cannot safely generate exact predicate for Opaque type\n"
+            "  consider generating a positive-predicate instead\n"
+            "  type: ~a\n"
+            "  pred: ~s\n")
+           type
            (syntax-e p?)))
         (flat/sc #`(flat-named-contract (quote #,(syntax-e p?)) #,p?))]
        [(Continuation-Mark-Keyof: t)