This patch adds a state to AppUpdater to represent when an internal error has occurred. It also adds the appropriate code to the Update UI to be able to handle this state, along with a panel in each UI and a string that can go in the panels to convey to the user that update has failed.
Ideally, it would be good if this message also directed the user to a SUMO article. See Bug 1787309 for details on this. But at the moment, I would prefer to address Bug 1756450 quickly rather than wait for that to be ready.
Differential Revision: https://phabricator.services.mozilla.com/D156998
ff6db15116
