#158: Remove fallback efforts for missing windows

minkphp/driver-testsuite#101 has introduced test coverage which expects an exception if we attempt to switch to a window which does not exist. chrome-mink-driver previously tried to handle this by dynamically creating popups with window.open(), but we will now throw a DriverException instead.

Related to #158 (closed)

@acoulton I'd be interested in your feedback on this change!