In a buffer with emacs-lisp-mode active, 'define-error' is not fontified
by font-lock mode.  The keyword should be listed somewhere around line 360 in
emacs-lisp/lisp-mode.el.

Recipe:

- Start emacs
- Visit a file ending in .el
- Type: (define-error 'new-error "this is a new error")
- Wait for fontification to complete and notice it is not highlighted

In lisp-mode.el, there is a comment about constructs being updated
automatically from obarray, but I was not able to understand how this
happens, and thought maybe the comment is referring to symbols created
with those commands?