add more tips about npe;

Author: zhenjun115

modules/swagger-codegen/src/main/java/io/swagger/codegen/DefaultGenerator.java

@@ -284,7 +284,7 @@ private Model getParent(Model model) {
                         allProcessedModels.put(name, models);
 
                     } catch (Exception e) {
-                        throw new RuntimeException("Could not process model '" + name + "'", e);
+                        throw new RuntimeException("Could not process model '" + name + "'" + ".Please make sure that your schema is correct!", e);
                     }
                 }