filer.copy: fail early if assign request failed

fix https://github.com/chrislusf/seaweedfs/issues/2216

weed/command/filer_copy.go

@@ -381,6 +381,9 @@ func (worker *FileCopyWorker) uploadFileAsOne(task FileCopyTask, f *os.File) err
 				if assignResult.Error != "" {
 					return fmt.Errorf("assign volume failure %v: %v", request, assignResult.Error)
 				}
+				if assignResult.Url == "" {
+					return fmt.Errorf("assign volume failure %v: %v", request, assignResult)
+				}
 				return nil
 			})
 		})

weed/operation/assign_file_id.go

@@ -73,6 +73,10 @@ func Assign(masterFn GetMasterFn, grpcDialOption grpc.DialOption, primaryRequest
 			ret.Error = resp.Error
 			ret.Auth = security.EncodedJwt(resp.Auth)
 
+			if resp.Error != "" {
+				return fmt.Errorf("assignRequest: %v", resp.Error)
+			}
+
 			return nil
 
 		})