CDB-2765 Gracefully finish geocoding if there are no rows to process
CDB-2765 Store a bit more info (needed by the browser)
CDB-2765 Give meaningful feedback to the user
CDB-2765 Fix tests and addition of a new one

Fix a few geocoding_spec.rb tests and add a new one for the case where
there are no rows to geocode.
Merge pull request #835 from CartoDB/CDB-2765

CDB-2765 - Geocoding a table with 0 geocodificable rows throws error