Return parser error location in a list

An error can have multiple locations which are returned in a listt with
key "locations".

tests/Language/GraphQL/ErrorSpec.hs

@@ -15,6 +15,6 @@ import Test.Hspec ( Spec
 spec :: Spec
 spec = describe "singleError" $
     it "constructs an error with the given message" $
-        let errors'' = Seq.singleton $ Error "Message." 0 0
+        let errors'' = Seq.singleton $ Error "Message." []
             expected = Response Aeson.Null errors''
          in singleError "Message." `shouldBe` expected