Facilitate re-authentication for 'configure' command

[MShekow]

At the moment, if the authentication (or authorization) for a calendar stops working (examples: a) the CalDAV password has changed; b) for Outlook 365, the client PC was offline for over one month, invalidating the OAuth refresh token), the user has to repeat the entire configuration process.

Instead, calendar providers should throw a specific exception that indicates that the authz/authn has become invalid, allowing the user to re-authenticate. A set of sensibly-chosen parameters should be re-used (e.g. O365 Client ID, or CalDAV username), offering them as default values, but still allowing the user to change them.

When the user then runs the configure command again, they are allowed to fix only the calendar authentication.