[llvm-objcopy][NFC] Propagate errors in removeSymbols/removeSectionReferences

Reviewers: jhenderson, alexshap, jakehehrlich, espindola

Reviewed By: jhenderson

Subscribers: emaste, arichardson, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D57543

llvm-svn: 352877