Cleanup OpenLayers geocoder.

Remove unneeded call to address.toProprietary(this.api).
Merge both requests, use a single variable for the Nominatim url address, inline the callback.
Move status check out of geocode_callback.
Fix error messages (because the server is running Nominatim, OpenLayers is used just to make requests).
Do not prepare the address string when not needed (when reverse geocoding is used).

Fix OpenLayers reverse geocoder broken by 6daf742 (pull request #202).

Pass the row_limit parameter to Nominatim.