buildAuthHandler returns incorrect error object

See https://github.com/couchbase/gocbcore/blob/341fc70dc8ba195416b4dc8f6d8599e391de686b/agent.go#L449-L466 if there is a callErr then we return the err from getKvAuthCreds (which will be nil). This could cause the consumer of the function to wait on either of the returned channels which will be nil and could lead to waiting on the channels forever.