XML Schema: Getting more information on validation failures?

Hello,

I'm new to lxml and I'm trying to get more information on why some 
validation is failing.  Here is the code I am currently using:

try:
   xmlschema.assertValid(xml_doc)
except etree.DocumentInvalid:
   traceback.print_exc()
   print log
   print error.domain_name
   print error.type_name
   sys.exit()


Here's the output I get:

Traceback (most recent call last):
 File "./xml.py", line 42, in ?
   xmlschema.assertValid(xml_doc)
 File "etree.pyx", line 1624, in etree._Validator.assertValid
DocumentInvalid: Document does not comply with schema

Traceback (most recent call last):
 File "./xml.py", line 46, in ?
   print error.domain_name
AttributeError: 'NoneType' object has no attribute 'domain_name'


Is there any way to get more information than this?
Thanks,

Adhamh