require explicit etherbase address for mining. Falling back to primary is...

require explicit etherbase address for mining. Falling back to primary is risky given it is inconsistent if keys are imported/merged/created or copied/transfered

eth/backend.go

@@ -464,10 +464,9 @@ func (s *Ethereum) StartMining(threads int) error {
 func (s *Ethereum) Etherbase() (eb common.Address, err error) {
 	eb = s.etherbase
 	if (eb == common.Address{}) {
-		err = fmt.Errorf("no accounts found")
-		return eb, err
+		err = fmt.Errorf("etherbase address must be explicitly specified")
 	}
-	return eb, nil
+	return
 }
 
 func (s *Ethereum) StopMining()         { s.miner.Stop() }