Remove PHP notices when memcache connection fails.

When Memcache can't be connected to, this API gracefully allows Drupal to continue without caching (which makes Drupal really slow, but still useful). The attached patch fixes a small PHP notice that occurs when Memcache connection fails. While small, this notice can appear in a drupal_set_message call which will make drupal_page_is_cacheable() return false which is less than optimal since with memcache down, your site is really going to slow down, you're going to want as much page level caching as possible (if not handled by memcache, like varnish for example).