Revert "bugpoint: Stop threading errors through APIs that never fail"

Revert "bugpoint: Stop threading errors through APIs that never fail"

This isn't the right thing to do - it turns out a number of the APIs
that "never fail" just exit(1) if something bad happens. We can and
should thread Error through this instead.

That diff will make more sense with this reverted. Sorry for the
noise.

This reverts r280690

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@280691 91177308-0d34-0410-b5e6-96231b3b80d8