Launching updater when reboot is still required results in error instead of reboot request

[eloquence]

Per @emkll's STR here

1. apply updates that require a reboot
2. close preflight updater
3. open preflight updater (no updates required), error is shown (because REBOOT_REQUIRED case is not handled, and is part of the catch-all)

Suggested fix:

The reboot_required case should be handled at this point and show the reboot button

securedrop-workstation/launcher/sdw_updater_gui/UpdaterApp.py

Line 113 in 87b2c61

else: