Minor refactor to checkProcessHasNotDied

To simplify the handling of maybe stderr.

hydra-test-utils/src/Test/Hydra/Prelude.hs

@@ -145,13 +145,13 @@ checkProcessHasNotDied name processHandle mStdErr =
   waitForProcess processHandle >>= \case
     ExitSuccess -> failure "Process has stopped"
     ExitFailure exit -> do
-      let failureMsg = "Process " <> show name <> " exited with failure code: " <> show exit
-      errorMsg <- case mStdErr of
-        Nothing -> pure [failureMsg]
-        Just stdErr -> do
-          errorOutput <- hGetContents stdErr
-          pure [failureMsg, "Process stderr: " <> errorOutput]
-      failure . toString $ unlines errorMsg
+      mErrorOutput <- traverse hGetContents mStdErr
+      let mErrorMsg = ("Process stderr: " <>) <$> mErrorOutput
+      failure . toString $
+        unlines
+          ( "Process " <> show name <> " exited with failure code: " <> show exit
+              : maybeToList mErrorMsg
+          )
 
 -- | Like 'coverTable', but construct the weight requirements generically from
 -- the provided label.