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.
Added some safeguards to throw an exception on invalid usage

A DriverException when invalid form actions are attempted on an element
is better than a fatal error.
Merge pull request #54 from stof/exception_class

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