687057 - Preferences could throw more useful error codes

Status: UNCONFIRMED

(Core :: Preferences: Backend, enhancement)

Description

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When getting a preference there are typically four possible errors:
1. The pref service hasn't been initialised
2. The preference does not exist
3. The preference only exists as a user preference (for the default branch)
4. The preference exists but is of the wrong type

The latter three throw NS_ERROR_UNEXPECTED which makes them harder to debug. Perhaps NS_ERROR_FAILURE or NS_ERROR_NOT_AVAILABLE could be used instead.