[llvm-objdump] - Remove one of `report_error` functions and improve the error reporting.

One of the report_error functions was taking object::Archive::Child as an
argument. It feels excessive, this patch removes it and introduce a helper
function instead. Also I fixed a "TODO" in this patch what improved the message printed.

Differential revision: https://reviews.llvm.org/D66468

llvm-svn: 369382