More informative error when a global reference is used in a context of
local variables which is a different from the one of its definition. E.g.:

Section A.
Variable n:nat.
Definition c:=n.
Goal True.
clear n.
Check c.

[I'm however unsure that "n" should not continue to be accessible via
some global (qualified) name, even after the "clear n".]

git-svn-id: svn+ssh://scm.gforge.inria.fr/svn/coq/trunk@16256 85f007b7-540e-0410-9357-904b9bb8a0f7
Repairing r16205: errors raised by check_evar_instance were no longer
trapped by solve_simple_eqn.

git-svn-id: svn+ssh://scm.gforge.inria.fr/svn/coq/trunk@16257 85f007b7-540e-0410-9357-904b9bb8a0f7