Add extra check to bot startup to ensure all roles within `STATISTICS_ROLES` list exists

This check will need to be performed in both the `on_ready` event, and also within the future command that will allow changing of config variables (likely to be `/config set `), implemented within #221.

It has to be called from the event/command side rather than from within the config variable loading process, because the config loading is done before the connection to Discord is made (so the existent roles could not be retrieved).