bug#20625: 25.0.50; doc of `define-error` is incorrect

[Drew Adams]

> > No, MESSAGE is not necessarily a string. It is a required argument, but
> > the code explicitly makes use of the case where it is nil.  And existing
> > code delivered with Emacs (e.g. bookmark.el) makes use of a nil MESSAGE.
> > That case should be documented.
> 
> The function doesn't error out if MESSAGE isn't a string, but it still
> shouldn't be.  So I don't think that's a bug.
> 
> Feel free to submit a new bug report for bookmark.el not supplying a
> string.

I think it is too bad that `define-error', which was intended to
be a friendly wrapper around using `error-conditions' directly,
will no longer do all that the latter does.  But so be it.

Bug #23408 reported, for bookmark.el.  Users who wrote
similar code, expecting `define-error' to be like using
`error-conditions', will need to likewise change their
code.