Fix GetTickerPrice() and GetCandlePrices() in price feeder providers to not return nothing if some pairs don't have price data

[rbajollari]

Summary of Bug

GetTickerPrice() and GetCandlePrices() will return no price data if even just one pair doesn't have price data. These methods should still return prices for pairs that have it available and log a warning about pairs that don't have price data, rather than completely error out when one pair doesn't.

Version

Please provide the output of the following commands:

• $ umeed version
• $ go version
• $ uname -a

Steps to Reproduce

Steps to reproduce the behavior:

1. ...

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