Bug 1552853 - Fix the update UI so it displays restart when it should when update staging fails. r=mhowell

Fixes the case reported where disabling the maintenance service caused the UI to display Applying due to the update staging failure.
Added new error codes for the two cases where pending was written to the update.status file when staging failed.
Added code to UpdateService.jsm to set the update.status to pending when either of the two new error codes are in the update.status file.
Added two new tests that verify the UI and simulate the failure condition in updater.cpp.

Differential Revision: https://phabricator.services.mozilla.com/D32177

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extra : moz-landing-system : lando