error message is more verbose when gate data is invalid

converter-CellDesigner/src/main/java/lcsb/mapviewer/converter/model/celldesigner/reaction/ReactionFromXml.java

@@ -564,6 +564,9 @@ public class ReactionFromXml extends XmlParser {
       num2 = points.size() - 1;
       result.getPoints().addAll(points);
     }
+    if (num0 == null || num1 == null) {
+      throw new InvalidXmlSchemaException("No information about line separation");
+    }
     result.setNum0(num0);
     result.setNum1(num1);
     result.setNum2(num2);