More specific information when no params/args specified

This commit adds the `ArrayIndexOutOfBoundsException` to onCommand's exception-catcher. This is so the user gets more specific information than "Error, check server.log" when they don't give any arguments.

Now, the user will receive "Not enough arguments given", which is a far more accurate and user-friendly error than "Error, check server.log" (which leads to "java.lang.ArrayIndexOutOfBoundsException: 1" in server.log).