Provide an internal function to add errors

The old function, addErrMsg, takes only a string with an error
description, but more information is required for the execution errors:
locations and path. addErrMsg should be deprecated after the switching
to the new addError.

src/Language/GraphQL/Execute/Internal.hs

@@ -0,0 +1,25 @@
+{- This Source Code Form is subject to the terms of the Mozilla Public License,
+   v. 2.0. If a copy of the MPL was not distributed with this file, You can
+   obtain one at https://mozilla.org/MPL/2.0/. -}
+
+{-# LANGUAGE DuplicateRecordFields #-}
+{-# LANGUAGE NamedFieldPuns #-}
+
+module Language.GraphQL.Execute.Internal
+    ( addError
+    ) where
+
+import Control.Monad.Trans.State (modify)
+import Control.Monad.Catch (MonadCatch)
+import Data.Sequence ((|>))
+import Language.GraphQL.Error
+import Language.GraphQL.Execute.Coerce
+import Prelude hiding (null)
+
+addError :: (Serialize a, MonadCatch m) => Error -> CollectErrsT m a
+addError error' = modify appender >> pure null
+  where
+   appender :: Resolution m -> Resolution m
+   appender resolution@Resolution{ errors } = resolution
+       { errors = errors |> error'
+       }