layered: Report wrong graph configuration instead of fixing it. (#790)

Signed-off-by: Soeren Domroes <sdo@informatik.uni-kiel.de>

plugins/org.eclipse.elk.alg.layered/src/org/eclipse/elk/alg/layered/ElkLayered.java

@@ -31,6 +31,7 @@
 import org.eclipse.elk.alg.layered.options.GreedySwitchType;
 import org.eclipse.elk.alg.layered.options.InternalProperties;
 import org.eclipse.elk.alg.layered.options.LayeredOptions;
+import org.eclipse.elk.core.UnsupportedGraphException;
 import org.eclipse.elk.core.alg.ILayoutProcessor;
 import org.eclipse.elk.core.math.KVector;
 import org.eclipse.elk.core.options.ContentAlignment;
@@ -336,7 +337,8 @@ private void reviewAndCorrectHierarchicalProcessors(final LGraph root, final Col
                 CrossingMinimizationStrategy childCms = 
                         child.getProperty(LayeredOptions.CROSSING_MINIMIZATION_STRATEGY);
                 if (childCms == CrossingMinimizationStrategy.LAYER_SWEEP) {
-                    child.setProperty(LayeredOptions.CROSSING_MINIMIZATION_STRATEGY, parentCms);
+                    throw new UnsupportedGraphException("The hierarchy aware processor " + childCms + " in child node "
+                            + child + " is only allowed if the root node specifies the same hierarchical processor.");
                 }
             });
         }