Better error handling for Geocoder enrich calls

This patches two bugs in the Geonames Geocoder:

  - Assumed geonames always returns an `fcode`. Now uses dict.get() to
  allow for variability in geonames results.
  - A bug in nested for loops caused continued operations on a place key
  after it had been removed in merge_related. This removes that conflict
  and better optimizes the nested loops.

This also introduces better error handling and logging for
Place.enrich_place, allowing for soft failover should of one Geocoder
error out.
Merge pull request #34 from dpla/geocoder-failover

Better error handling for Geocoder enrich calls