Add status string for unknown status codes

status.go

@@ -1,5 +1,7 @@
 package ftp
 
+import "fmt"
+
 // FTP status codes, defined in RFC 959
 const (
 	StatusInitiating    = 100
@@ -109,5 +111,9 @@ var statusText = map[int]string{
 
 // StatusText returns a text for the FTP status code. It returns the empty string if the code is unknown.
 func StatusText(code int) string {
-	return statusText[code]
+	str, ok := statusText[code]
+	if !ok {
+		str = fmt.Sprintf("Unknown status code: %d", code)
+	}
+	return str
 }