Improve error message when argument regex is not valid

src-hxbrief/Main.hs

@@ -759,13 +759,13 @@ main = B.mainFromCmdParser $ do
             let tx = t x
             regexE <- PCRE.compile PCRE.compBlank PCRE.execBlank tx
             case regexE of
-              Left  err   -> error $ show err
+              Left  err   -> error $ "failed parsing regex '" ++ x ++ "': " ++ show err
               Right regex -> pure (joinMode, tx, regex)
           compilerStop x = do
             let tx = t x
             regexE <- PCRE.compile PCRE.compBlank PCRE.execBlank tx
             case regexE of
-              Left  err   -> error $ show err
+              Left  err   -> error $ "failed parsing regex '" ++ x ++ "': " ++ show err
               Right regex -> pure regex
       compiled_summarize <-
         sequence