Improve handling of Currency update responses

[cameronsstone]

There are two commits here:

1. Generically reject any currency updates unless both are true:
   1. the currency is in CURRENCIES or CRYPTO_CURRENCIES
   2. the rate is a number.
2. Explicitly handle throttling responses from Cryptocompare.com's API which unfortunately use a HTTP 200 instead of a 429 or similar.

This should help with #74 .