follow-up https://github.com/spesmilo/electrum/pull/8536
This replaces 69336befee, which was insufficient.
#8536 added a new field into the struct, which older versions do not ignore but raise:
opening a wallet file with new code updated the struct to include it,
after which old code could no longer open the wallet file.
i.e. #8536 was an invisible wallet upgrade, breaking compat.
This commit simply formalises the wallet upgrade: old code will now show
an understandable error when trying to open new files.
cee22abcb5