Change API to return error on 500 http status.

- The Telegram API does not return well-formed JSON when there is an 
Internal Server Error. This results in incomprehensible errors.

api.go

@@ -74,6 +74,10 @@ func sendFile(method, token, name, path string, params url.Values) ([]byte, erro
 		return []byte{}, err
 	}
 
+	if resp.StatusCode == http.StatusInternalServerError {
+		return []byte{}, fmt.Errorf("Telegram API returned 500 internal server error")
+	}
+
 	json, err := ioutil.ReadAll(resp.Body)
 	if err != nil {
 		return []byte{}, err