[llvm-readobj]Fix error messages for bad archive members and add testing for archive handling

llvm-readobj's error messages were broken for bad archive members. This
patch fixes them, and also adds testing for archive and thin archive
handling within llvm-readobj.

Reviewed by: rupprecht, grimar, higuoxing

Differential Revision: https://reviews.llvm.org/D58681

llvm-svn: 354960