Add more information to assertion

git-svn-id: https://svn.apache.org/repos/asf/poi/trunk@1871593 13f79535-47bb-0310-9956-ffa450edef68

src/ooxml/java/org/apache/poi/xssf/usermodel/XSSFRow.java

@@ -254,7 +254,7 @@ public class XSSFRow implements Row, Comparable<XSSFRow> {
                 cell.setCellErrorValue(FormulaError._NO_ERROR);
                 break;
             default:
-                throw new AssertionError();
+                throw new AssertionError("Unknown cell-type specified: " + type);
         }
     }