Handle invalid Comet Cache Pro credentials gracefully

[raamdev]

In wpsharks/comet-cache-pro#272 we improved the Pro Upgrader by using the build-in WordPress upgrade system. However, if the wrong credentials are entered in the Comet Cache options, or if you've entered credentials and then cleared the credentials after a new version of Comet Cache Pro has been detected, attempting to update Comet Cache Pro results in the following confusing and misleading error:

We can probably hook into upgrader_pre_download before the download is attempted to verify the credentials are correct, and then return our own WP_Error() when they're not.