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.)