Port Syntaxerr.Error to the new system. Trickier, because of special way to report some errors in the toplevel (is it really worth the trouble?).

git-svn-id: http://caml.inria.fr/svn/ocaml/branches/exception_registration@14105 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02