args to error, cerror, & warn

[Glenn S. Burke <GSB @ MIT-ML>]

I think we should require a first argument of NIL there now for
a place-holder since we are not going to define the extension yet.
Optional first arguments are a pain for both implementation and
documentation.
(Many of us have been doing this for years with FERROR in maclisp
anyway, where it was never really well-defined what a non-null
first argument meant.)