bugpoint: Stop threading errors through APIs that never fail

This simplifies ListReducer and most of its subclasses by removing the
std::string &Error that was threaded through all of them but almost
never used. If we end up needing error handling in more places here we
can reinstate it using llvm::Error instead of these unwieldy strings.

The 2 cases (out of 12) that actually can hit the error cases are a
little bit awkward now, but those will clean up as I refactor this API
further.

llvm-svn: 280690