Fixed the error handling in the driver

This makes the driver pass the new tests of https://github.com/Behat/Mink/pull/505

Among the broken tests, 2 of them were failing with the wrong exception (a LogicException coming from the DomCrawler layer) and a bunch of other tests were triggering a fatal error when trying to use ``null`` as crawler node for invalid XPath (I don't know exactly how many, as a fatal error halts the suite).