ar: provide error exit status upon failure

Previously ar and ranlib returned with exit status 0 (success) in the
case of a missing file or other error. Update to use error handling
similar to that added by ELF Tool Chain after that project forked
FreeBSD's ar.

PR: PR257599 [exp-run]
Reported by: Shawn Webb, gehmehgeh (on HardenedBSD IRC)
Reviewed by: markj
Obtained from: elftoolchain
MFC after: 2 months
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D31402