Update Vzaar.php

[bjrhodes]

Update to whoami request to make the plugin a little more resilient and informative on developer error.

I am using variable configs depending on situation and had some uninformative errors because of inconsistent responses from the API on error. I've just thrown in a couple of more specific exceptions when the whoami falis, as this is likely to be first port-of-call for config and plugin integration.

[andheiberg]

Thanks man :) apparently this package isn't even versioned, so I won't go through and add versions. I hope your changes doesn't break anything ;D