error: Report Secret Service errors

Don't crash when unable to save session; show in-app notifications when either session saving or session restoring fails.

Fix #796 (closed). Partial fix #808 (closed), because we still don't report when the error was due to a Secret Service provider not being installed (because I still don't know what the error message in that case actually is).

The strategy used with the boolean variable is a little bad. One reason is that we have to wait until the prepare signal is emitted to use parent_window(), but that control-flow dependency is only visible in the comment I left.

However, the only alternative I can think of is doing the plumbing to somehow be able to call append_error on the window at that location. I think that means either wiring up Result<> return types back up to Login::login() (async stuff would be required as well), or wiring access to the Login widget down to handle_login_result. I don't want to do either of those things, but I would be willing to if that's deemed a superior approach.