message: issue/17 Slightly more explicit error message
diff: @@ -240,7 +240,7 @@ impl IndexMerger {
  .field_type()
  .get_segment_postings_option()
  .expect("Encounterred a field that is not supposed to be
  - indexed. Have you modified the index?");
  + indexed. Have you modified the schema?");
  last_field = Some(current_field);
  // it is perfectly safe to call `.new_field`
  