Add document about error cells (#4)

editor/customCells.md

@@ -48,6 +48,7 @@ If you want to store the node as its actual node type (`node<MyConcept>`) within
 
 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.
+Additionally, this technique can be used to create error cells, as described in [Error Cells](./errorCells.md).
 
 It is also possible to set the style on those cells:
 

editor/errorCells.md

@@ -0,0 +1,44 @@
+# 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.
+
+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;
+  }
+
+  @Override
+  public EditorCell createEditorCell(EditorContext p1)  {
+    new  EditorCell_Error(p1, this.getSNode(), "Error: " + this.theNode.someBehavior(), false);
+  }
+};
+```
+
+The final boolean parameter of the constructor is called `editable`.
+The [javadoc] says the following:
+```
+@param editable - there are two different kinds of CEll_Error in MPS:
+ - one (!editable) intended to show error text and then substitute it completely then user type
+   something e.g. list<|<no type>>
+ - another (editable) allows editing error text directly without replacing it with first types character
+   e.g. myVariable.|field - in case "field" is not resolved, it should be highlighted as error, but should
+   be still completely editable
+```
+So if `false`, the cell will completely be replaced by the first character the user types, if `true` the content may be edited.
+
+Even though the cell is an error cell and also sets the internal error state to true,
+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>`):
+
+![../media/screenshot_error_cell.png](../media/screenshot_error_cell.png)
+
+[javadoc]: https://github.com/JetBrains/MPS/blob/7ed0ccb451197c7dc33d9395dbe0dfa74ca96786/editor/editor-runtime/source/jetbrains/mps/nodeEditor/cells/EditorCell_Error.java#L35-L42