throw exception on failed edge adding

src/main/java/kieker/analysis/graph/impl/ExceptionFactory.java

@@ -33,6 +33,10 @@ class ExceptionFactory {
 		return new IllegalStateException("Edge with id does not exist: " + id);
 	}
 
+	public static IllegalStateException verticesAreNotInSameGraph(final Object id1, final Object id2) {
+		return new IllegalStateException("Vertices with ids are not in the same graph: " + id1 + ", " + id2);
+	}
+
 	public static IllegalArgumentException bothIsNotSupported() {
 		return new IllegalArgumentException("A direction of BOTH is not supported");
 	}

src/main/java/kieker/analysis/graph/impl/GraphImpl.java

@@ -89,8 +89,7 @@ public class GraphImpl extends ElementImpl implements Graph {
 		}
 
 		if (outVertexParents.peek().graph != inVertexParents.peek().graph) {
-			// throw exception TODO
-			throw new IllegalStateException();
+			throw ExceptionFactory.verticesAreNotInSameGraph(outVertex.getId(), inVertex.getId());
 		}
 
 		GraphImpl firstEqualParent = null;