fix: GetTickerPrice() and GetCandlePrices() in providers still returns available ticker or candle data if not all currency pairs error

[rbajollari]

Description

closes: #24

• Added two new error types in errors.go for when a provider has no ticker or candle data.
• If any individual currency pair can't return a ticker or candle, GetTickerPrice() and GetCandlePrices() will log a warning for that pair and only return an error back if all the currency pairs it was asked to return return an error.
• Updated provider tests to check for new error message returned from GetTickerPrice() and GetCandlePrices()
• Create new example config for Forex rates and get rid of Forex pairs in original config since an API key is required for that config to work

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[zarazan]

Looks good, one comment regarding the error numbers