filer.copy: don't crash when volume creation fails

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x28 pc=0x1d58247]

goroutine 7482 [running]:
github.com/chrislusf/seaweedfs/weed/command.(*FileCopyWorker).uploadFileInChunks.func1(0x2)
        /go/src/github.com/chrislusf/seaweedfs/weed/command/filer_copy.go:488 +0x2a7
created by github.com/chrislusf/seaweedfs/weed/command.(*FileCopyWorker).uploadFileInChunks
        /go/src/github.com/chrislusf/seaweedfs/weed/command/filer_copy.go:455 +0x225
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x28 pc=0x1d58247]

goroutine 7480 [running]:
github.com/chrislusf/seaweedfs/weed/command.(*FileCopyWorker).uploadFileInChunks.func1(0x0)
        /go/src/github.com/chrislusf/seaweedfs/weed/command/filer_copy.go:488 +0x2a7
created by github.com/chrislusf/seaweedfs/weed/command.(*FileCopyWorker).uploadFileInChunks
        /go/src/github.com/chrislusf/seaweedfs/weed/command/filer_copy.go:455 +0x225

weed/command/filer_copy.go

@@ -482,7 +482,8 @@ func (worker *FileCopyWorker) uploadFileInChunks(task FileCopyTask, f *os.File,
 				})
 			})
 			if err != nil {
-				fmt.Printf("Failed to assign from %v: %v\n", worker.options.masters, err)
+				uploadError = fmt.Errorf("Failed to assign from %v: %v\n", worker.options.masters, err)
+				return
 			}
 
 			targetUrl := "http://" + assignResult.Location.Url + "/" + assignResult.FileId