Have a "Forced Update" option for non-backward compatible Snap updates.

Sometimes Snap builders have an update to their Snap which is not backward compatible to the previous versions, either in the Snap code or the Snap dependencies code. When an end-user is prompted to update the Snap and they chose not to update, they face a broken user-experience.

To avoid such a scenario, it would be nice to have a "Forced" Update prompt to the end-user. If the user chooses not to update, they would not be able to continue to use the Snap. Only a successful update of the Snap would allow the end-user to proceed with using the Snap.

No files, tests, or entry points are named in the issue. Start by locating the Snap update prompt and the code that permits continued use after a declined update; define the affected update states and verify that only a successful update permits use.

Written by the indexing model from the issue text.