Add updater error messages

[Dentomologist]

Fixes error reporting during update checks and adds error messages for several things that can go wrong while triggering an update.

On master, any error during an update check either incorrectly reports Dolphin is up to date (for manual checks) or is just logged (for automatic checks). This PR reports the correct errors, and also shows a message box for all manual check results and certain automatic results to make issues more visible.

Event details:

• Up to date: (Unchanged from master, only shown for manual checks):
  "You are running the latest version available on this update track."
• Check request failed: (Error, manual):
  "Unable to contact update server."
• Invalid JSON in server response: (error, manual and automatic):
  "Invalid JSON received from auto-update service: {error description}."
• Updater copy failed: (error, manual and automatic)
  "Unable to create updater copy."
• Failed to set permissions: (error, manual and automatic)
  "Unable to set permissions on updater copy."
• Updater launch failed: (error, manual and automatic)
  "Could not start updater process: {error description}."

Discussion items:

• Wording of error messages.
• Which errors during automatic checks should generate an alert.

[Pokechu22]

Would SuccessAlertFmt/CriticalAlertFmt be usable here? That system is already UI frontend agnostic.

[Dentomologist]

Pokechu22: Thank you, I didn't know about those.

[shuffle2]

lgtm

[AdmiralCurtiss]

There's an unused bool m_update_available in the Updater header now, from what I can tell.

Otherwise yeah seems fine.