Cerrors: get rid of some FIXME

git-svn-id: svn+ssh://scm.gforge.inria.fr/svn/coq/trunk@16304 85f007b7-540e-0410-9357-904b9bb8a0f7
Embedded exns in LtacLocated and EvaluatedError satisfy Errors.noncritical

 This was apparently already the case before, but this invariant is now
 explicited in comments + a few asserts.

git-svn-id: svn+ssh://scm.gforge.inria.fr/svn/coq/trunk@16305 85f007b7-540e-0410-9357-904b9bb8a0f7