[llvm-objcopy][NFC] More error cleanup

Summary:
This removes calls to `error()`/`reportError()` in the main driver (llvm-objcopy.cpp) as well as the associated argv-parsing (CopyConfig.cpp). `logAllUnhandledErrors()` is now the main way to print errors.

There are still a few uses from within the per-arch drivers, so we can't delete them yet... but almost!

Reviewers: jhenderson, alexshap, espindola

Reviewed By: jhenderson

Subscribers: emaste, arichardson, jakehehrlich, jdoerfert, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D58316

llvm-svn: 354600