[llvm-objcopy] Improve error message for unrecognised archive member

Prior to this patch, llvm-objcopy's error messages for archives with
unsupported members only mentioned the archive name, not the member
name, making them unhelpful. This change improves it by approximately
following GNU objcopy's error message syntax of
"<archive name>(<member name>): <problem>".

Reviewed by: grimar

Differential Revision: https://reviews.llvm.org/D61674

llvm-svn: 360251