jsonStorage will throw exception if key not found

[oliverwoodings]

This is the code for the 'get' method of jsonStorage:

    get: function (key) {
      var raw = getStorage().getItem(getNamespacedKey(key));
      try {
        var payload = JSON.parse(raw);
        return payload.value;
      } catch (e) {
        throw new Error('Unable to parse JSON from storage');
      }
    }

If a key is not defined, this will throw an exception saying cannot parse, even though the issue is actually that it will throw a 'cannot read property value of payload'.

Really it should just return null if raw is null. I'll do a PR.