Handle 'stale element' as element not existing

Occasionally webdriver throws an error about the element reference being stale.
Let's handle that case as the element doesn't yet exist. All other errors are re
thrown as before.

Note: I don't have any tests for this as I cannot yet reliably reproduce the state. It's Webdriver throwing the error, and apparently when the DOM is manipulated while waiting for the element to become visible. I'll keep working on a reproducable test case.