Don't issue an error when Feeds plugin is missing, but a warning.

feeds.module

@@ -605,7 +605,7 @@ function feeds_plugin_instance($plugin, $id) {
 
   // Return FeedsMissingPlugin if no plugin was found.
   // @todo Better error handling.
-  drupal_set_message(t('Missing Feeds plugin. Check whether all required libraries and modules are installed properly.'), 'error');
+  drupal_set_message(t('Missing Feeds plugin. Check whether all required libraries and modules are installed properly.'), 'warning');
   $class = ctools_plugin_load_class('feeds', 'plugins', 'FeedsMissingPlugin', 'handler');
   return FeedsConfigurable::instance($class, $id);
 }