Add warning to prevent panic when password storage isn't unblocked or even installed

flatpak --env=RUST_BACKTRACE=full run org.gnome.FractalNext.Devel
Jun 22 07:16:32.529  INFO fractal::application: Fractal (org.gnome.FractalNext.Devel)    
Jun 22 07:16:32.529  INFO fractal::application: Version: 0.0.1-8fdf61d4 (Devel)    
Jun 22 07:16:32.529  INFO fractal::application: Datadir: /app/share/fractal    
Jun 22 07:16:32.530 DEBUG fractal::application::imp: GtkApplication<Application>::startup    
Jun 22 07:16:32.532 DEBUG fractal::application::imp: GtkApplication<Application>::activate    
Jun 22 07:16:32.558  WARN fractal::window: Failed to restore previous sessions: NoResult    
thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: NoResult', src/session/mod.rs:311:52