What steps will reproduce the problem?
1. Use an Open ID Provider that supports only stateless.
2. Use CustomerManager.associate to try to associate (it is not always 
known that the Open ID Provider only supports stateless, so trying to 
associate makes sense).
3. The Open ID Provider returns an error response with error_code and 
error parameters, see http://openid.net/specs/openid-authentication-
2_0.html 8.2.4
4. Message.validate throws an Exception that assoc_type is a required 
field because it is listed in the required fields of AssocationResponse

What is the expected output? What do you see instead?
An exception is thrown an logged, this is unnecessary: assoc_type isn't a 
required field for association responses, only for successful 
associations. ConsumerManager will however use stateless and continues.

What version of the product are you using? On what operating system?
0.9.5 on Windows XP.

Please provide any additional information below.
AssociationResponse should be able to accept unsuccessful association 
responses. It should have a method to check whether or not the response 
was successful.