CoqIDE: do not try to backtrack to a dummy id

git-svn-id: svn+ssh://scm.gforge.inria.fr/svn/coq/trunk@16906 85f007b7-540e-0410-9357-904b9bb8a0f7
CoqIDE: always retag on insert

This should fix Arnaud's bug (reported by private email) that makes
coq eat two sentences at once (and process only the first one).

git-svn-id: svn+ssh://scm.gforge.inria.fr/svn/coq/trunk@16907 85f007b7-540e-0410-9357-904b9bb8a0f7
STM: do not enrich exceptions more than once

git-svn-id: svn+ssh://scm.gforge.inria.fr/svn/coq/trunk@16908 85f007b7-540e-0410-9357-904b9bb8a0f7
STM: proper error message if the GUI edits_at a dummy state

git-svn-id: svn+ssh://scm.gforge.inria.fr/svn/coq/trunk@16909 85f007b7-540e-0410-9357-904b9bb8a0f7