Try both sources when downloading currency data

[printfn]

The UN exchange rates endpoint is currently down so currency conversions don't work. We should try fetching data from both endpoints to make fend more resilient.

[Markos-Th09]

Yeah, I think aggregating data could improve the exchange rate results, as the ECB updates more often for major currencies. Also if we are willing to cache data on our own from rate limited exchange rate APIs we could get something more reliable.