catch failures of pr_vernac to make -time failsafe

Printing Ltac code is broken. This was not compiling with -time

  Ltac pop_succn e := eval lazy beta iota delta [plus] in (1 + 1).

The bug in the pretty printer should be fixed too...

git-svn-id: svn+ssh://scm.gforge.inria.fr/svn/coq/trunk@16262 85f007b7-540e-0410-9357-904b9bb8a0f7