Don't barf() on failures in loadArchive()

This patch replaces calls to barf() in loadArchive() with proper
error handling.

Test Plan: GHC CI

Reviewers: rwbarton, erikd, hvr, austin, simonmar, bgamari

Reviewed By: bgamari

Subscribers: thomie

Tags: #ghc

Differential Revision: https://phabricator.haskell.org/D2652

GHC Trac Issues: #12388