Investigate a smoother UX for `isMortalEra` option when decoding

[TarikGul]

rel: #332

If a user decides to decode a transaction without any knowledge of the mortal era, they might not know that they have to input isImmortalEra as an option if it was used to construct the transaction. We should investigate a smoother UX for the user that can potentially make that usecase easier and without error.