Improve error message printing

- Omit unnecessary show-invocation
- Use showOutputable for the error span (location)

before/after:

  "RealSrcSpan SrcSpanPoint \"stdin\" 2 1: parse error (possibly incorrect indentation or mismatched brackets)"
  stdin:2:1: parse error (possibly incorrect indentation or mismatched brackets)

src-brittany/Main.hs

@@ -325,7 +325,7 @@ coreIO putErrorLnIO config suppressOutput inputPathM outputPathM =
     case parseResult of
       Left left -> do
         putErrorLn "parse error:"
-        putErrorLn $ show left
+        putErrorLn left
         ExceptT.throwE 60
       Right (anns, parsedSource, hasCPP) -> do
         (inlineConf, perItemConf) <-

src/Language/Haskell/Brittany/Internal/ExactPrintUtils.hs

@@ -123,7 +123,7 @@ parseModuleFromString args fp dynCheck str =
     dynCheckRes <- ExceptT.ExceptT $ liftIO $ dynCheck dflags1
     let res = ExactPrint.parseModuleFromStringInternal dflags1 fp str
     case res of
-      Left  (span, err) -> ExceptT.throwE $ show span ++ ": " ++ err
+      Left  (span, err) -> ExceptT.throwE $ showOutputable span ++ ": " ++ err
       Right (a   , m  ) -> pure (a, m, dynCheckRes)