Use xbt_ex_setup_backtrace to actually setup the backtrace to avoid dupplicating...

Use xbt_ex_setup_backtrace to actually setup the backtrace to avoid dupplicating this code; avoid the word 'break' within a TRY block, even if it should be ok here (rest of a badly paranoid debuging party); deal with fact that a callback not being attached to a RPC can raise exceptions too (in which case, we should propagate it)

git-svn-id: svn+ssh://scm.gforge.inria.fr/svn/simgrid/simgrid/trunk@2117 48e7efb5-ca39-0410-a469-dd3cf9ba447f