fix: panic when torrent not exist

pkg/doc.go

@@ -1,14 +1,14 @@
 package pkg
 
 type Torrent interface {
-	Name() string
-	Progress() int
+	Name() (string, error)
+	Progress() (int, error)
 	Stop() error
 	Start() error
 	Remove() error
 	Save() string
 	Exists() bool
-	SeedRatio() *float64
+	SeedRatio() (float64, error)
 }