Introduce a surf_parse_error function to deal with parse errors in the input file...

Introduce a surf_parse_error function to deal with parse errors in the input file: user don't care about the code stack in that context, but want to spot the error in the file. Also, change the brain dead code to deal with foreach radical attribute to a half decent one (at least, we try to do the right thing)

git-svn-id: svn+ssh://scm.gforge.inria.fr/svn/simgrid/simgrid/trunk@5797 48e7efb5-ca39-0410-a469-dd3cf9ba447f