* tokens.ts: improve documentation
Improve variable naming and documentation on the methods in `tokens.ts`.
* rename restoreFromLocalStorage
Since the session data isn't actually stored in localstorage, this feels like a
misleading name.
* Lifecycle: bail out if picklekey is missing
Currently, if we have an accesstoken which is encrypted with a picklekey, but
the picklekey has gone missing, we carry on with no access token at all. This
is sure to blow up in some way or other later on, but in a rather cryptic way.
Instead, let's bail out early.
(This will produce a "can't restore session" error, but we normally see one of
those anyway because we can't initialise the crypto store.)
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