inline Locale.prototype.setReady
expose readyState on mozL10n
return from ctx.ready early if the ctx is ready
guard public get* methods from running if ctx is not ready

otherwise, they end up creating new locales if called too early, and then
building it with no resources (b/c the ctx is not ready)
abstract the fallback loop into ctx.getWithFallback
don't throw when not ready
if missing, return an empty string