build failure proper report.

src/Argonix.hs

@@ -419,8 +419,9 @@ wrap nixCommand target sa@ArgsCommon {..} = do
                     (P.proc (toS nixc) $ fmap toS arglist)
           )
         >>= \case
-              ExitSuccess   -> copySuccess "/tmp/papa"
-              ExitFailure _ -> copyFailure $ toS tmpXDGHOME
+              ExitSuccess -> copySuccess "/tmp/papa"
+              ExitFailure _ ->
+                copyFailure (toS tmpXDGHOME) >> die "build failed."
  where
   copySuccess source =
     runProcessVerbose verbosity "cp" ["-r", source, "./result"] []