[llvm-objcopy] Changed command line parsing errors

Summary: Tidied up errors during command line parsing to be more consistent with the rest of llvm-objcopy errors.

Reviewers: jhenderson, rupprecht, espindola, alexshap

Reviewed By: jhenderson, rupprecht

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D62973

llvm-svn: 363350