Copyright (c) 2012 Kevin Wellwood
All rights reserved.
This source code is distributed under the Modified BSD License. For terms and
conditions, see license.txt.
Returns True if the enemy is shootable (meaning it blocks shots fired at
it.) Being shootable does not imply that the enemy will react in any
specific way if instructed to die.
Notify the enemy that something deadly happened to it and it should die.
If the enemy is not currently capable of dying, nothing should happen.
The default behavior is to do nothing. Override this procedure to cause
the enemy to react in some way.