Handle no seat (remote desktop sessions)

[Weilbyte]

If the user is trying to open the application while on a session with no seat (e.g. remote desktop), the session monitor will fail retrieving the /org/freedesktop/login1/seat/auto object (because of an unknown object exception from D-Bus) and this exception is not caught. This way, exception handler shows unknown error with no information.

This PR implements an exception for this (seemed cleaner this way than checking for dbus.exceptions.DBusException in exception handler directly), and implements logic to the exception handler to catch it and provide the user with information regarding the exception (and a workaround with manual connection).

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This can be reproduced by having a VM with VNC server, and then connecting to it with a VNC client and starting the Proton VPN client from the same session.