requireFile: exit with non-zero error message

Since the script running is a failure condition, we should fail the
build properly, not leaving it up to the missing output to determine
that the build went wrong.  This should partly address #38952 — nix
build will print out the build log on non-zero exits.

pkgs/build-support/trivial-builders.nix

@@ -170,6 +170,7 @@ rec {
         ***
 
         _EOF_
+        exit 1
       '';
     };