Make it clear who is responsible for rejecting messages

[jakoschiko]

In some cases imap-next is actively rejecting messages (e.g. imap-next is sending a BAD in case of Error::LiteralTooLong). In other cases the user is responsible for that. The responsibility is not well documented.

Another problem is that the user has no direct access to the tag. We could improve the situation by adding the tag to the error types. Maybe we could even use this for documentation: If the error type contains a tag, then the user is responsible for sending BAD.