write to stderr when no maven wagon was found

Otherwise, this problem is completely invisible,
ending up as "cannot resolve maven artifact" only.

bundles/org.eclipse.tea.library.build/src/org/eclipse/tea/library/build/tasks/maven/SynchronizeMavenArtifact.java

@@ -555,7 +555,9 @@ private static Wagon newOfFirstWagonAvailable(String... classnames)
 			if (clazz != null) {
 				return clazz.newInstance();
 			}
-			throw new ClassNotFoundException("one of " + String.join(", ", classnames));
+			ClassNotFoundException e = new ClassNotFoundException("one of " + String.join(", ", classnames));
+			e.printStackTrace();
+			throw e;
 		}
 
 		@Override