[C] Validate Server method input. Addresses BUKKIT-3687

##### The Issue:
Server methods do not check their parameters. Plugins can give pass in null pointers, to the methods causing a NullPointerException. [Bukkit-3687](https://bukkit.atlassian.net/browse/BUKKIT-3687) is caused by a plugin passing in null to a method. Resulting in a null pointer exception being thrown inside CraftBukkit methods.

##### Justification for this PR:
This causes methods to fail early and identify that a plugin is responsible for passing in an invalid argument. This will only change the exception thrown, if there originally was a thrown  exception. This helps with hunting down legitimate problems with CraftBukkit.

##### PR Breakdown:
CraftServer methods that implement the Server interface will throw an IllegalArgumentException if a method cannot operate on a null input and given a null pointer.

##### Relevant PR(s):
N/A

##### JIRA Ticket:
BUKKIT-3687 - https://bukkit.atlassian.net/browse/BUKKIT-3687