add code

ytdl/error.go

@@ -0,0 +1,26 @@
+package ytdl
+
+type Error struct {
+	// `Input` is the input given to youtube-dl when the error occurred.
+	// If `Input` is set to "", it is ignored.
+	Input string
+	// `Err` is the underlying error.
+	Err error
+}
+
+// `input` may be set to "", in which case it is ignored when outputting the
+// error message.
+func newError(input string, err error) *Error {
+	return &Error{
+		Input: input,
+		Err:   err,
+	}
+}
+
+func (e *Error) Error() string {
+	if e.Input == "" {
+		return "ytdl: " + e.Err.Error()
+	} else {
+		return "ytdl['" + e.Input + "']: " + e.Err.Error()
+	}
+}