Multiple address-balance request returns zero BTC balances due to rate limit

[msgilligan]

When querying the new /v2/address/addr/ Omniwallet can return a zero balance for BTC, because the upstream web-service providing this information is rate-limited.

At a minimum, when this error condition occurs on the "upstream" request, an error result should be returned the the Omniwallet client request.

Ideally, Omniwallet will be able to return this information in a way that the rate limits are not triggered.