mega: fix unmarshal error on download - FIXME need to upstream this fix

Failed to copy: failed to open source object: open download file
    failed: json: cannot unmarshal number -18 into Go struct field
    DownloadResp.e of type uint32

See: https://forum.rclone.org/t/failed-message-what-does-it-mean/9065

vendor/github.com/t3rm1n4l/go-mega/mega.go

@@ -894,6 +894,11 @@ func (m *Mega) NewDownload(src *Node) (*Download, error) {
 		return nil, err
 	}
 
+	// DownloadResp has an embedded error in it for some reason
+	if res[0].Err != 0 {
+		return nil, parseError(res[0].Err)
+	}
+
 	_, err = decryptAttr(key, []byte(res[0].Attr))
 	if err != nil {
 		return nil, err

vendor/github.com/t3rm1n4l/go-mega/messages.go

@@ -123,7 +123,7 @@ type DownloadResp struct {
 	G    string   `json:"g"`
 	Size uint64   `json:"s"`
 	Attr string   `json:"at"`
-	Err  uint32 `json:"e"`
+	Err  ErrorMsg `json:"e"`
 }
 
 type UploadMsg struct {