bugpoint: Return Errors instead of passing around strings

This replaces the threading of `std::string &Error` through all of
these APIs with checked Error returns instead. There are very few
places here that actually emit any errors right now, but threading the
APIs through will allow us to replace a bunch of exit(1)'s that are
scattered through this code with proper error handling.

This is more or less NFC, but does move around where a couple of error
messages are printed out.

llvm-svn: 280720