gRpc connection error on filer when no volume left #2243

fix https://github.com/chrislusf/seaweedfs/issues/2243

grpc do not cache connections only when connection problem happens.
Normal error results should not close the shared grpc connection.

weed/pb/grpc_client_server.go

@@ -119,9 +119,7 @@ func WithCachedGrpcClient(fn func(*grpc.ClientConn) error, address string, opts
 	}
 	executionErr := fn(vgc.ClientConn)
 	if executionErr != nil {
-		vgc.errCount++
-		if vgc.errCount > 3 ||
-			strings.Contains(executionErr.Error(), "transport") ||
+		if strings.Contains(executionErr.Error(), grpc.ErrServerStopped) ||
 			strings.Contains(executionErr.Error(), "connection closed") {
 			grpcClientsLock.Lock()
 			if t, ok := grpcClients[address]; ok {