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.
Could not reproduce on my laptop. That "staling" happened between `element(loc)` and `.is_displayed` which is weird enough.