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).
If there is a runtime exception you are not expecting it will be consumed. You should also not be doing nothing with it. I understand why you've done it, but it is bad practice. Java 7 has multi-exception catch support, should suffice.