added error codes to the stages

src/main/java/teetime/stage/InitialElementProducer.java

@@ -29,7 +29,7 @@ public class InitialElementProducer<T> extends AbstractProducerStage<T> {
 
 	public InitialElementProducer(final Iterable<T> elements) {
 		if (elements == null) {
-			throw new IllegalArgumentException("The given iterable must not be null");
+			throw new IllegalArgumentException("4002 - The given iterable must not be null.");
 		}
 		this.elements = elements;
 	}

src/main/java/teetime/stage/ObjectProducer.java

@@ -37,7 +37,7 @@ public final class ObjectProducer<T> extends AbstractProducerStage<T> {
 	 */
 	public ObjectProducer(final long numInputObjects, final ConstructorClosure<T> inputObjectCreator) {
 		if (numInputObjects < 0) {
-			throw new IllegalArgumentException("numInputObjects must be non-negative.");
+			throw new IllegalArgumentException("4001 - numInputObjects must be non-negative.");
 		}
 		this.numInputObjects = numInputObjects;
 		this.inputObjectCreator = inputObjectCreator;