Add support for cells in error when writing XLSX and ODS

When appending data to an existing sheet, it was possible to get cells in error when reading (DIV/0 for instance). When trying to write them back, `addRow` would throw because `Cell`s in error were not supported by the writers.

src/Spout/Common/Entity/Cell.php

@@ -91,6 +91,14 @@ class Cell
         return !$this->isError() ? $this->value : null;
     }
 
+    /**
+     * @return mixed
+     */
+    public function getValueEvenIfError()
+    {
+        return $this->value;
+    }
+
     /**
      * @param Style|null $style
      */