add a clearErrors() and clearMessages() method to ActionSupport

Currently ActionSupport has a clearErrorsAndMessages() method. I think it will be useful to be able to clear errors and/or messages individually. It's not always the case that you want to clear both. See the message thread below:

http://www.nabble.com/-s2--no-result-defined-error-after-failed-validation---can-someone-help--tf4030311.html

I believe the user wants to be able to clear errors and not necessarily messages. I have found a need for this myself recently and would like to implement or see it implemented by someone else.

I would commit the code myself to do this, but it requires an update to the XWork code. Not sure what the process is to get XWork code updated.