Changed exception to use a DriverException when the XPath does not match

The NotFoundElementException is meant to be used by higher level APIs (the Element API), not by drivers themselves.
This makes the driver consistent as some checks were using DriverException and some others were using NotFoundElementException.

Refs https://github.com/Behat/Mink/issues/467 and https://github.com/Behat/Mink/issues/473