Enable failure on warnings in epydoc

This causes epydoc to fail on any warning.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>

epydoc.conf

@@ -25,3 +25,5 @@ inheritance: listed
 
 parse: yes
 introspect: no
+
+fail-on: docstring_warnings