[bugpoint] Report non-existent opt binary

Bugpoint will keep going even if the opt binary it's given doesn't
exist. It should at least alert the user, so it's clear why reductions
are failing.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@324713 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed