Nicer error message when using slot-boundp on nil

[Zach Beane]

(slot-boundp nil 'foo) produces a nice error message:

  When attempting to
  test to see whether slot is bound (SLOT-BOUNDP), the slot FOO is
  missing from the object NIL.

Well, the line-breaking is odd, but it's pretty clear what it means.

However, in the non-constant case, the message is different:

  (let ((foo nil)) (slot-boundp foo 'bar)) =>
  There is no applicable method for the generic function
    #<STANDARD-GENERIC-FUNCTION
      (SB-PCL::SLOT-ACCESSOR :GLOBAL BAR BOUNDP) (1)>
  when called with arguments
    (NIL).

It took me a while to figure out what it meant.

[Zach Beane (xach)]

It seems like this is not just for NIL, but for any kind of non-CLOS-instance object. (I was confused by the NIL case, though.)