try to get more info about the HTML that appears to fail assertions

Author: albu-diku

tests/support/htmlsupp.py

@@ -68,4 +68,4 @@ def assertIsValidHtmlDocument(self, value, permit_no_close=False):
         except Exception as exc:
             error = exc
         if error:
-            raise AssertionError("failed to verify input string as HTML: %s", str(error))
+            raise AssertionError("failed to verify input string as HTML: %s\ndocument:%s" % (str(error), value))