Error checkers

This Pull Request relies on rewrite in https://github.com/watir/watir-webdriver/pull/282

Current behavior throws an exception if error checker makes a driver call on a window that has been closed
This code does not run error checkers if current window is closed

Current behavior throws an exception if error checker makes a driver call when an alert is present.
This code adds methods to interact with alert (via block) and close it before error checkers are run. This is also advantageous since accepting an alert can update the window, after which we should be running error checkers, anyway.
The issue is backward compatibility. Our current alerts specs will all fail without the #empty? check here, because the wire call closes the alert in Firefox.
Right now, it is impossible to run an alert test and have any error checker that makes a wire call, so we need a #without_checkers or #delays_checkers regardless.
Because there is never a time when the window the driver is pointing to has closed that you would want to run the error checkers, so it seems preferable to check for the window than to require using a special method.
The only important use case for #without_checkers is dealing with alerts because they are tricky buggers. maybe #delay_checkers does make more sense to keep the intent more clear or usage limited?