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

[Drew Adams]

> > In fact, I think that the right change would be not only
> > to say what nil MESSAGE means but to make MESSAGE an
> > &optional parameter.
> 
> I disagree.  The bookmark.el code is just a historical accident.

Then please remove it.  Either this is an intended feature
or it is not.  If you don't want `define-error` to provide
exactly the same thing that you can get by using property
`error-conditions' directly, then leave its definition and
doc as they are, but change the `bookmark.el' code back to
using `error-conditions' directly, so any code that expects
error condition `bookmark-errors' to be available will not
be surprised.  Or remove it from bookmark.el altogether,
if you don't care about such backward compatibility.  Or
provide a MESSAGE arg for the definition of `bookmark-errors'.

One way or another, the doc and behavior should match, and
bookmark.el should be made coherent with both.