Terser logging

The development logging is currently a bit overwhelming. This PR trims it down a bit, à la 39046680a.
> > Will we still see messages about conflicting existing mappings?

> [Yes](https://github.com/openannotation/annotator-store/blob/3943d448c114884e76fbf97707e1172cc3ff0127/annotator/elasticsearch.py#L113).

The `reindex.py` file, suggested in the error message you referenced above, does not exist in our repo.
So it's not exactly helpful for suggesting the users to use it....