Improve test coverage for invalid formula handling

Author: sanshi42

tests/samples/invalid_formula.xls

@@ -79,3 +79,12 @@ def test_if(self):
 
     def test_choose(self):
         self.assertEqual(self.get_value(1, 6), "'C'")
+
+    def test_evaluate_name_formula_with_invalid_operand(self):
+        book = xlrd.open_workbook(from_sample('invalid_formula.xls'))
+        sheet = book.sheet_by_index(0)
+        cell = sheet.cell(0, 0)
+
+        self.assertEqual(cell.ctype, xlrd.XL_CELL_ERROR)
+        self.assertIn(cell.value, xlrd.error_text_from_code)
+