GDBusMethodInvocation misuse throughout user.c
In !51 (comment 421955), Ray noticed that we need to return TRUE in the AccountsUserIface vfuncs to handle the signal, rather than FALSE to continue to run other handlers. When we return FALSE, _accounts_user_skeleton_handle_method_call() will call g_dbus_method_invocation_return_error(), which is bad because (a) we already did, in our throw_error(), and (b) that unrefs invocation, which we also already did.