Improve errorCells documentation (#5)

Also add sentence to customCells documentation

README.md

@@ -11,3 +11,4 @@ This repository contains some tips and tricks around JetBrains MPS, which I miss
 ### Editor
 
 - [Custom cells and Swing Components](./editor/customCells.md)
+- [Custom Error Cells](./editor/errorCells.md)

editor/customCells.md

@@ -44,7 +44,7 @@ Then you can instantiate an anonymous subclass of `AbstractCellProvider`:
 ```
 Alternatively you can also create a named subclass of `AbstractCellProvider` in an util model and instantiate that.
 
-If you want to store the node as its actual node type (`node<MyConcept>`) within the Provider, you can also override the constructor with a typed argument, store in a typed class attribute and pass it down to the super constructor.
+If you want to store the node as its actual node type (`node<MyConcept>`) within the Provider, you can also override the constructor with a typed argument, store in a typed class attribute and pass it down to the super constructor. Alternatively, you can also store the node in a final attribute of the class (does not need constructor and therefore works in anonymous class).
 
 To be able to instantiate cells, you usually want to import the model `jetbrains.mps.nodeEditor.cells@java_stub`.
 I've found that the cells `EditorCell_Constant` and `EditorCell_RefPresentation` are particularly useful.

editor/errorCells.md

@@ -1,22 +1,20 @@
-# Error Cells
+# Custom Error Cells
 
 Sometimes it is desired to make a cell of your editor appear like a usual error as automatically generated by MPS, e.g. when a property is empty which is not allowed to be empty.
-This can be for example be the case in the editor of an abstract concept or as the empty cell of a reference or child cell.
+
+Usually it is enough to use the provided `CellModel_Error` aka `[!<text>!]` aka `error`.
+However, this currently only supports static text and is therefore not sufficient in all cases.
+For example if you want to customize the empty cell of a child or reference cell with text about what this cell is about (e.g. parameter name, etc).
 
 To do this we can use a custom cell provider as described in [Custom Cells](./customCells.md#custom-cell) with a `EditorCell_Error`:
 
 ```
 new AbstractCellProvider(node) {
-  private node<myConcept> theNode;
-
-  public AbstractCellProvider$anonymous(node<MyConcept> theNode)  {
-    super(theNode);
-    this.theNode = theNode;
-  }
+  private final node<MyConcept> theNode = node;
 
   @Override
   public EditorCell createEditorCell(EditorContext p1)  {
-    new  EditorCell_Error(p1, this.getSNode(), "Error: " + this.theNode.someBehavior(), false);
+    new  EditorCell_Error(p1, this.getSNode(), this.theNode.getMyErrorString(), false);
   }
 };
 ```
@@ -37,7 +35,7 @@ Even though the cell is an error cell and also sets the internal error state to
 this does not cause an error during model check.
 Therefore, it is necessary to also check the appropriate condition in a checking rule.
 
-The result can look like this (both the `<no type>` and the `<none>`):
+The result can look like this (the `<no type>` is a usual error cell, but the parameter placeholders are custom):
 
 ![../media/screenshot_error_cell.png](../media/screenshot_error_cell.png)