Bug 130854 [CellEditors] JFace TableViewer ignoring ICellEditor validator state
Using a validator on a TextCellEditor and entering invalid text into a cell results in ICellModifier.modify() being called with a null value, rather than an error being reported and the value being discarded. TableEditorImpl.saveEditorValue() *does actually check for editor validity*, but in the case of invalid values does nothing but no-op with comment "///Do what ???" and then calls cellEditor.getValue() which, for TextCellEditor's anyway, always returns null if its validator has invalidated the current value.
bundles/org.eclipse.jface/src/org/eclipse/jface/viewers/ColumnViewerEditor.java
if (nextCell != null) {
			if (isCellEditable(viewer, nextCell)) {
				rv = nextCell;
			} else {
				rv = searchNextCell(row, nextCell, originalCell, viewer);
			}
		} else {
			if ((feature & TABBING_CYCLE_IN_ROW) == TABBING_CYCLE_IN_ROW) {
				rv = searchNextCell(row, null, originalCell, viewer);
			} else if ((feature & TABBING_MOVE_TO_ROW_NEIGHBOR) == TABBING_MOVE_TO_ROW_NEIGHBOR) {
				ViewerRow rowBelow = row.getNeighbor(ViewerRow.BELOW, false);
				if (rowBelow != null) {
					rv = searchNextCell(rowBelow, null, originalCell, viewer);
				}
			}
		}
		return rv;
	}
	/**
	 * Position the editor inside the control
	 *
	 * @param w
	 *            the editor control
	 * @param item
	 *            the item (row) in which the editor is drawn in
	 * @param fColumnNumber
	 *            the column number in which the editor is shown
	 */
	protected abstract void setEditor(Control w, Item item, int fColumnNumber);
	/**