Gaia: how to check that login was successful?

Apologies if I've missed something obvious, but how should I check that a login, e.g. from `login_gui()` was successful? The method does not return anything to indicate success or failure.

It looks like I'd need to check `Gaia._GaiaClass__gaiatap._TapPlus__isLoggedIn`, which seems less than ideal given that it is using two levels of name-mangled "private" variables.

Start by reading the Gaia.login_gui() entry point and the TapPlus login state referenced in the issue. Exercise the login flow to establish the current success and failure behavior. Done means users have a documented, public way to determine whether login succeeded without accessing name-mangled private variables.

Written by the indexing model from the issue text.