[llvm-objcopy][MachO] Print an error message on use of unsupported options

Summary:
It is better to print an error message instead of silently ignoring unsupported options.

As mentioned in https://reviews.llvm.org/D57045, this is not the best solution and we should print which flag is not supported at some time.

Reviewers: alexshap, rupprecht, jhenderson, jakehehrlich

Reviewed By: alexshap, rupprecht, jakehehrlich

Subscribers: jakehehrlich, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D62578

llvm-svn: 362040