IndexError in get_rate() method

Hello guys. Thank you for the work you are doing.

Wanted to report a small bug to you. It looks like our BTCPay node is not very stable sometimes or there are some networking issues. For some reason `client.get_rate("USD")` occasionally may throw an `IndexError list index out of range` exception.

Based on the info from our Sentry dashboard I do see that `get_rates()` method returned an empty list and then further, on line 97 it will brake, because it tries to access the first element of the empty list `rates = []` : https://github.com/btcpayserver/btcpay-python/blob/master/btcpay/client.py#L97

Not sure if you like to raise another error in that case, but just decided to report it here.

Thanks!