Flesh out request API with relevant failure errors

[nathanhosselton]

So can do things like

firstly {
    Bungie.searchForClan(named: "Meow Pew Pew")
}.then { clan in
    Bungie.getClanMembers(in: clan)
}.done { members in
    //…
}.catch { error in
    switch error {
    case Bungie.Error.clanNotFound:
        //e.g. bounce the search bar
    default:
        //show generic message
    }
}