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.

**I could not reproduce this on my laptop**
As we discussed on IRC, this can happen on observed fields (they are replaced by new elements).