Improve handling of when users subscribe to a full workbook

[mcoles]

Users will often subscribe to an entire Workbook rather than a single sheet. VizAlerts ignores this completely at present, which is confusing for users who expect their alert to run.

At minimum, we should improve this by throwing an error when the alert is run on its schedule, and inform the user that they need to pick a single sheet, rather than the whole workbook.

Better behavior might be processing all sheets in the workbook as their own individual VizAlerts, if you subscribe to the workbook as a whole. But this seems like it wouldn't be a common use case, especially with people building content references and things like that. Even Conditional Subscriptions doesn't support this right now. If you build a ton of Simple Alerts, it could be somewhat useful. But my guess is that the majority of users would just be confused about the number of emails they were pelted with.