Harden util::prefs setting/resetting

[frewsxcv]

Right now, it's very easy to make typos with key names when working with util::prefs. In an attempt to make the system less-error-prone, I propose that setting (util::prefs::set_pref) and resetting (util::prefs::reset_pref) only work on keys that already exist in the underlying map structure. If they don't exist, we either panic! or return Result::Err (I don't have strong opinions between these two options). If adding new keys at runtime (that don't exist in resources/prefs.json) is something we need to support, I suggest there be a separate function called new_pref or something similar. Thoughts?