Kraken: Fix TestGetOpenInterest (#1611)

* Kraken: Fix TestGetOpenInterest

We see daily failures on OpenInterest for Kraken.
This fix assumes that the issue might be related to volume of ETHUSD
open interest, and switches the single pair test to XBTUSD instead

Also isolates the test from others, since we're changing stored pairs
and we might be colliding on the global k

* Kraken: Handle Errors field in futures response

common/common.go

@@ -67,6 +67,7 @@ var (
 	ErrCannotCalculateOffline = errors.New("cannot calculate offline, unsupported")
 	ErrNoResponse             = errors.New("no response")
 	ErrTypeAssertFailure      = errors.New("type assert failure")
+	ErrUnknownError           = errors.New("unknown error")
 )
 
 var (