issue #1467 avoid error when Piwigo version is not available on PEM

* do not display an error message "cannot connect to server" but simply "no plugin available"
* if we are on a beta/RC version, display a button to switch to test plugins compatible with Piwigo previous version
* add a tooltip with more details about the "show beta test plugins"

admin/plugins_new.php

@@ -181,5 +181,10 @@ else
   $page['errors'][] = l10n('Can\'t connect to server.');
 }
 
+if (!$beta_test and preg_match('/(beta|RC)/', PHPWG_VERSION))
+{
+  $template->assign('BETA_URL', $base_url.'&beta-test=true');
+}
+
 $template->assign_var_from_handle('ADMIN_CONTENT', 'plugins');
 ?>