Improve the semantic error when += to non-abstract

[danchanka]

The erroneous code:

CLASS Chat;
someAbstractProperty = DATA VARSTRING[100] (Chat);  // not abstract
someProperty = DATA VARSTRING[100] (Chat);
someAbstractProperty(Chat chat) += someProperty(chat);

At the moment, the error text is "property 'someAbstractProperty' not found.". It would be better to have "You can += only to abstract property" or "Property 'someAbstractProperty' is not abstract".

Related issue: lsfusion/plugin-idea#25