Make DetectorError print something in python2

Also works for python3, we call __init__ of the base class.
Hopefully fixes issue2551057.

Author: schlatterbeck

roundup/cgi/exceptions.py

@@ -29,6 +29,7 @@ def __init__(self, subject, html, txt):
         self.subject = subject
         self.html = html
         self.txt = txt
+        BaseException.__init__ (self, subject + ' ' + txt)
 
 class FormError(ValueError):
     """An 'expected' exception occurred during form parsing.