Deprecate internal error generation functions

The functions generating errors in the executor should be changed anyway
when we provide better error messages from the executor, with the error
location and response path. So public definitions of these functions are
deprecated now and they are replaced by more generic functions in the
executor code.

src/Language/GraphQL/Error.hs

@@ -69,21 +69,25 @@ parseError ParseErrorBundle{..}  =
 type CollectErrsT m = StateT (Resolution m) m
 
 -- | Adds an error to the list of errors.
+{-# DEPRECATED #-}
 addErr :: Monad m => Error -> CollectErrsT m ()
 addErr v = modify appender
   where
     appender :: Monad m => Resolution m -> Resolution m
     appender resolution@Resolution{..} = resolution{ errors = errors |> v }
 
+{-# DEPRECATED #-}
 makeErrorMessage :: Text -> Error
 makeErrorMessage s = Error s [] []
 
 -- | Constructs a response object containing only the error with the given
 -- message.
+{-# DEPRECATED #-}
 singleError :: Serialize a => Text -> Response a
-singleError message = Response null $ Seq.singleton $ makeErrorMessage message
+singleError message = Response null $ Seq.singleton $ Error message [] []
 
 -- | Convenience function for just wrapping an error message.
+{-# DEPRECATED #-}
 addErrMsg :: (Monad m, Serialize a) => Text -> CollectErrsT m a
 addErrMsg errorMessage = (addErr . makeErrorMessage) errorMessage >> pure null