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**
Do we plan to export this as a constant for use elsewhere? If not, I don't think we need all caps, and I do think a leading underscore is great. Based on its usage below, I'd prefer to see this as just a lonely number 2 in the function next to a comment that explains why we chose 2.