Outdated firmware error messages were originally raised from
create_client, which would mean that a client for an outdated device
would not be created.
This had a number of undesirable outcomes due to "client does not exist"
being conflated with "no device is connected".
Instead, we raise in setup_client (which prevents creating new wallets
with outdated devices, BUT shows them in device list), and python-trezor
also raises on most calls (which gives us an error message when opening
wallet and/or trying to do basically anything with it).
This is still suboptimal - i.e., there's currently no way for Electrum to
claim higher version requirement than the underlying python-trezor, and
so minimum_firmware property is pretty much useless ATM.
