Check for an existing cert before renewing

[cambell-prince]

If an existing cert exists the plugin attempts to issue and may well get an "expansion required" error, e.g. attempting to add www to an already issues subdomain cert.

If the plugin were to check for the existence of a cert and continue on it would be more robust.

[cambell-prince]

The plugin now supports 'expansion' so this is less of an issue.