Plug error when issue #14 happens.

Also refactor code and comments a bit.
Only do overspill when instigator is Entity

There are certain damage events, such as "force" from strats, that don't carry
over an entity as instigator. In these events, the the instigator is the
damageData the projectile instead.

Closes #14.
Merge pull request #30 from FAForever/fix-nil-instigator

Fix nil instigator