fix moon destruction bug

[Jbaukens]

fix problem #181 there should probably be a better code but atleast it work, so you might adapt it and you know what the problem was

Regards

[jkroepke]

HI!

I will decline this pull request, because this problem does exist on all missions. We need a more generic way instead modify a single mission file.

[Jbaukens]

Hi i do agree and i do not agree
The problem could happen on all mission but destruction mission is the only where a planet can disipaer when something is going to all others missions (attack spy...) can not happen because you are not allowed to delete a planet when you have incomming fleets
Regards

[jkroepke]

Normally, this should be never happens. If the target moon/planet does not exist, it should raise an exception.
The bug you fixed in #181 avoid this problem, too.

It is always better to raise exceptions instead silence them away. Or we need to implement a log system in 2moons.