Fixing #2968. This is quite brittle though, because we are messing
with the exception catching system of Ltac which is not really
known for its safety, so that it may break a few things.

git-svn-id: svn+ssh://scm.gforge.inria.fr/svn/coq/trunk@16411 85f007b7-540e-0410-9357-904b9bb8a0f7