find-face is incompatible with XEmacs version

[Glenn Morris]

The new function find-face is incompatible with the XEmacs version,
and therefore breaks existing package code that tries to handle both
Emacs flavours.

The GNU Emacs version throws an error for an unknown face:

(find-face 'made-up-face)

whereas the XEmacs version simply returns nil. This breaks code such
as the following (from vm-startup.el):

(if (if (fboundp 'find-face)
        (find-face 'vm-monochrome-image)
      (facep 'vm-monochrome-image))
    nil
  (make-face 'vm-monochrome-image)
  (set-face-background 'vm-monochrome-image "white")
  (set-face-foreground 'vm-monochrome-image "black"))