normalize error message for mem{ber|q|v}

Restore roughly the old error message for BC, but also adjust it to
use the modern error-message style (with newlines).

racket/collects/racket/private/member.rkt

@@ -5,7 +5,8 @@
   ;; helper for member error cases
   (define-values (bad-list)
     (λ (who orig-l)
-      (raise-mismatch-error who "not a proper list: " orig-l)))
+      (raise-arguments-error who "not a proper list"
+                             "in" orig-l)))
 
   (define-values (member)
     (letrec-values ([(member)

racket/src/bc/src/list.c

@@ -1742,7 +1742,7 @@ scheme_checked_list_ref(int argc, Scheme_Object *argv[])
 static void mem_past_end(const char *name, Scheme_Object *s_arg, Scheme_Object *arg)
 {
   scheme_contract_error(name,
-                        "reached a non-pair",
+                        "not a proper list",
                         "in", 1, arg,
                         "looking for", 1, s_arg,
                         NULL);

racket/src/cs/rumble/error-rewrite.ss

@@ -111,7 +111,7 @@
                            "  in: ~s")
             irritants)]
    [(or (eq? who 'memq) (eq? who 'memv))
-    (values "not a proper list: ~s" irritants)]
+    (values "not a proper list\n  in: ~s" irritants)]
    [(equal? str  "~s is not a valid index for ~s")
     (cond
      [(exact-nonnegative-integer? (car irritants))