Fix typo, improve debuggung

src/main/java/de/uni/bremen/grapa/library/graphs/implementation/directed/BasicDirectedGraph.java

@@ -171,10 +171,10 @@ public class BasicDirectedGraph implements DirectedGraph<Vertex> {
     @Override
     public DirectedEdge addEdge(final Vertex start, final Vertex end) {
         if (!vertexMap.containsKey(start.getId())) {
-            throw new IllegalArgumentException(start + " is not in the Graph");
+            throw new IllegalArgumentException(start + " (start) is not in the Graph");
         }
         if (!vertexMap.containsKey(end.getId())) {
-            throw new IllegalArgumentException(end + " is not in the Graph");
+            throw new IllegalArgumentException(end + " (end) is not in the Graph");
         }
         DirectedEdge edge = new BasicDirectedEdge(start, end);
         edgesSet.add(edge);