checkout: throw an error when called with empty $station

			my $user     = $self->get_user_status( $uid, $db );

			my $train_id = $user->{train_id};

			if ( not $station ) {

				$self->app->log->error("Checkout($uid): station is empty");

				return ( 1, 'BUG: Checkout station is empty.' );

			}

			if ( not $user->{checked_in} and not $user->{cancelled} ) {

				return ( 0, 'You are not checked into any train' );

			}