Improve error message when trying to load group after rotating key

[cardoso]

Currently, it just says "Decryption failed". I think we need to be more descriptive, like: "Loading group failed because this user's current private key cannot decrypt the group key. If this user rotated their private key, they must be added to the group again."

To reproduce: https://github.com/VirgilSecurity/demo-e3kit-ios/tree/error_load_group_rotate
Branch: error_load_group_rotate