fix: Improve error message when an iframe was removed

[AdnoC]

After trying many things, this is the best we can do. When a StaleElementReferenceError we have a reference to an element that was either deleted or detached from the DOM. And we shouldn't be doing anything that could cause either of those things to occur. If a user's page causes Selenium to throw a StaleElementReferenceError then we our Axe results probably won't be consistent anyways.

So, the best solution is to inform the user that it is their page that is acting up and that it is their responsibility to get the page in a stable state before running Axe on it.

Closes issue: https://github.com/dequelabs/attest-node-suite/issues/246

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