Summary:
Apparently `sysErrorBelch` doesn't terminate the program anymore making
previously unreachable code now execute. If a dll is not found the error
message we return needs to be a heap value.

Secondly also allow the pattern `lib<name>` to be allowed for finding an
import library with the name `lib<name>.dll.a`.

Test Plan: ./validate, new tests T13082_good and T13082_fail

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

Reviewed By: RyanGlScott, bgamari

Subscribers: thomie, #ghc_windows_task_force

Differential Revision: https://phabricator.haskell.org/D2941

GHC Trac Issues: #13082