Try to protect from StaleElementReferenceException in is_displayed

I noticed that this happened in last Jenkins build in `test_ldap_auth_and_roles.py`.

If the passed locator is retrievable multiple times (string, ...), it is tried multiple times. Ignored with WebElement, as it is already broken if it raises StaleElementException.
It should be. The thing is that in that short period, some DOM change must have occured. So, it is probable that it won't happen again.