Fix for exceptions handling in HtmlOutput’s formatErr method

Now it should no longer print internal errors instead of intended ones
on Python 3.

Thanks to Maurice van der Pot for finding this fix.

tests/plugins.py

@@ -97,6 +97,8 @@ def finalize(self, result):
 
     def formatErr(self, err):
         exctype, value, tb = err
+        if not isinstance(value, exctype):
+            value = exctype(value)
         return ''.join(traceback.format_exception(exctype, value, tb))
 
     def startContext(self, ctx):