bug#14705: 24.3.50; "Not a face" error from enable-theme

[Sebastian Wiesner]

"enable-theme" signals a "Not a face" error if the color theme tries to
set a face which is currently not defined.  In "emacs -Q"

M-: (setq debug-on-error t)
M-: (load-theme 'wombat :no-confirm)

gives the following back trace:

Debugger entered--Lisp error: (error "Not a face: message-separator")
  signal(error ("Not a face: message-separator"))
  error("Not a face: %s" message-separator)
  check-face(message-separator)
  face-spec-recalc(message-separator #<frame F1 0x10181c238>)
  custom-theme-recalc-face(message-separator)
  enable-theme(wombat)
  load-theme(wombat :no-confirm)
  eval((load-theme (quote wombat) :no-confirm) nil)
  eval-expression((load-theme (quote wombat) :no-confirm) nil)
  call-interactively(eval-expression nil nil)
  command-execute(eval-expression)

This breaks *any* color theme, even the ones included in Emacs, because
all themes define faces which do not exists, e.g. because the
corresponding package is not yet loaded, or not even installed.