[llvm-objcopy][NFC] More error propagation

Summary: Do some more error cleanup, removing some dependencies from llvm-objcopy's error/reportError in [ELF/COFF]Objcopy methods.

Reviewers: jhenderson, alexshap, jakehehrlich, mstorsjo, espindola

Subscribers: emaste, arichardson

Differential Revision: https://reviews.llvm.org/D57423

llvm-svn: 352625