cmd/server: offer bypass for data source errors

[adamdecaf]

#248 (and previous issues) bring up the fact that Watchman should offer a way to temporarily ignore errors in parsing/downlaod of data files. As we grow the number of sources it's more likely at least one is serving invalid data or experiencing downtime.

Watchman serves the old files if download/parsing fails, but is unable to refresh any data when one source is down.

What would this look like in config? IGNORE_OFAC_SDN_COMMENTS=yes ?

ofac:
  allowRefreshFailure:
     sdn_comments: true