replace exit() by throw sg_exception(). Of course, we have to be aware

replace exit() by throw sg_exception(). Of course, we have to be aware
that interdependencies between sg libs are generally unwelcome, but
sg_exception is a rather basic part, and it's already used by xml, props,
scene, sound and, of course, structure. Since props and xml are core
libs, we can assume that sg_exceptions are available.  (OK'ed by Curt)