Instead of maintaining a list of script execution result types, which we
then map to WebDriver error types, just return the WebDriver error that
is specified by the spec. Then perform the JSON clone algorithm from the
caller in WebDriverConnection, again as specified by the spec. To do so,
this moves the JSON clone algorithm to its own file. This will also be
the future home of the JSON deserialize algorithm, which will need some
of the internal AOs implemented there.
