Discovery returns null certificate if connection blocked

[ccamacho1966]

If an underlying firewall/IPS blocks the attempted connection to the front-end, a null certificate object will be returned to Venafi. The application will be dropped, but we should detect this issue and not return the bad entry at all. We should also log the event if debugging is turned on.

It would be nice if we could somehow indicate this discovery error without aborting the entire discovery process... Not sure how I would go about that. Inserting events is non-trivial and would require additional Venafi API usage/configuration.