Any exception thrown on missing element action will make tests pass

Related to Behat/Mink#578
Looking once more in SahiClient code (e.g. `$this->client->findByXPath($xpath)->getText()`) the `findByXPath` operation doesn't really find anything and it's the `getText` operation that will be very surprised when it notices attaching files to checkboxes.

This leaves me with no choice but to set INVALID_EXCEPTION to `Exception` as you're suggesting.