Favor Pp.msg_warn (with flush) over Pp.warning (without), fixing #3013

 Without flush, the warning could arrive *after* an error message,
 leading ProofGeneral to think that the last sentence has been successful.

git-svn-id: svn+ssh://scm.gforge.inria.fr/svn/coq/branches/v8.4@16437 85f007b7-540e-0410-9357-904b9bb8a0f7