Reworked the command registration system

Please read the [javadoc](https://github.com/Limeth/Terasology/blob/16965803d4edd128ce5baaa3c35e814de26752f3/engine/src/main/java/org/terasology/logic/console/Command.java).
It is a bad practice to catch Throwable (can cause maintenance issues if api changes to have new caught exceptions, or later you want a caught exception to bubble up). You should catch RuntimeException and other specific exceptions.