Log at warn level when setup or tear down fails

Hi Ian,

Another small patch!

When setup or tear down fails I've switched the logging from debug to warn.  That way if the system environment isn't setup properly (e.g. missing webdriver.ie.driver property) we are told straight away rather than rather than having to switch logging to debug to find out why.

I've also taken the opportunity to tidy up the slf4j logging by using parameterised methods instead.

Regards,

Peter.