Restrict try...with to avoid interfering with Sys.Break

 This is a restricted version of the patchs applied recently on
 trunk. Idea: catching-all try...with are now restricted via
 a "when Errors.noncritical e" clause. In the 8.4 branch, this
 Errors.noncritical function only excludes the Sys.Break (and
 Out_ot_memory and Stack_overflow), but not the anomalies, for
 maximal compatibility.

 This commit concerns only coqtop and its plugins
 (no checker / ide / tools).

git-svn-id: svn+ssh://scm.gforge.inria.fr/svn/coq/branches/v8.4@16313 85f007b7-540e-0410-9357-904b9bb8a0f7