[llvm-objcopy][NFC] refactor error handling. part 1.

Remove usages of special error reporting functions(error(),
reportError()). This patch is extracted from D87987.
Errors are reported as Expected<>/Error returning values.
This part is for MachO subfolder of llvm-objcopy.

Testing: check-all.

Reviewed By: jhenderson, alexshap

Differential Revision: https://reviews.llvm.org/D88113