refreshment/last_update dates not handled correctly

[guidorice]

This issue collects a couple of related things about the handling of last refreshed/last updated dates in scoreboard and osmesa:

1. In api/src/services/osmesa.js getUpdates() Date.now() is returned if the osmesa refreshments table is empty. This could (possibly) lead to a situation where scoreboard ui is displaying the current date is shown but really it is unknown if/when the database last updated. It would be better to throw an exception and let the UI display an empty value or error message.

2. The the dev environment, the osmesa refreshments table is empty, as is the scoreboard user_update table. In #530 I listed some sql queries to populate these tables for testing in development. These tables should be added to the seed data in scoreboard and in https://github.com/developmentseed/osm-fabricator so it just works in the dev environment.