Improve errors for forbidden ops on weak sets

Forbidden set operations on weak sets produced slightly confusing error
messages:

    > (define s (weak-set))
    > (set-add s 42)
    ; set-add:
    ; expected: (not/c set-mutable?)
    ; given mutable set: (weak-set)
    ; argument position: 1st
    > (set-mutable? s)
    #f

Invoking an operation that required an immutable set gave a message
saying that a `(not/c set-mutable?)` was expected, but a `weak-set` is
not recognized by `set-mutable?`, which only recognizes mutable sets
with strongly-held keys. Changing `set-mutable?` to recognize weak sets
seems undesirable.

This patch changes the error message so that the above example produces
the following error:

    > (define s (weak-set))
    > (set-add s 42)
    ; set-add:
    ; expected: (not/c (or/c set-mutable? set-weak?))
    ; given mutable set: (weak-set)
    ; argument position: 1st

racket/collects/racket/private/set-types.rkt

@@ -682,7 +682,7 @@
                     "given ~a: ~e\n"
                     "argument position: 1st")
      method-name
-     (if immut "(not/c set-mutable?)" "set-mutable?")
+     (if immut "(not/c (or/c set-mutable? set-weak?))" "(or/c set-mutable? set-weak?)")
      (if immut "mutable set" "immutable set")
      s)
     (current-continuation-marks))))