Fix #9438 by converting a panic to an error message

Previously, GHC was quite eager to panic whenever it was fed
an archive file when `DYNAMIC_GHC_PROGRAMS=YES`. This ought to be an
explicit error message instead, so this patch accomplishes just that.

Test Plan: make test TEST=T14708

Reviewers: Phyx, hvr, bgamari

Reviewed By: Phyx

Subscribers: thomie, carter

GHC Trac Issues: #9438, #14708, #15032

Differential Revision: https://phabricator.haskell.org/D4589