Return Selenium own Java Exception on Failed Cmd.

Selenium server assumes that in the JSON response
of a failed command, the “class” field returns the
fully qualified name of the Java Exception the
Server is supposed to map the error to.

A very “cavalier” behaviour that I have changed 
in Selenium (see https://code.google.com/p/selenium/source/detail?r=4959695b2267deb16d829290d47c73e829c59422).
But, in the meantime, I decide I’d make GhostDriver
comply: so far I haven’t really used that field
to report anything useful, so it’s OK to change.

Fixes #354.