Log clearer errors when picklekey goes missing (#27)

* 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.)

src/components/structures/MatrixChat.tsx

@@ -367,7 +367,7 @@ export default class MatrixChat extends React.PureComponent<IProps, IState> {
             // Create and start the client
             // accesses the new credentials just set in storage during attemptDelegatedAuthLogin
             // and sets logged in state
-            await Lifecycle.restoreFromLocalStorage({ ignoreGuest: true });
+            await Lifecycle.restoreSessionFromStorage({ ignoreGuest: true });
             await this.postLoginSetup();
             return;
         }