as the default is now the empty string "".
It should fix the problem that we have lost the assertion messages
(seeing "what(): basic_string::_S_construct NULL not valid" instead),
for packages that use package-specific assertion macros.
* CGAL_error to CGAL_error_msg
* introduced a macro CGAL_error()
* added some words about CGAL_error to the developers manual
* renamed most of assert(x) into CGAL_assertion(x)
* renamed exit(x) with x != 0 , CGAL_assertion(false) and assert(false) into CGAL_error
* CORE left untouched, OpenNL changed