Good news. BOL is going to include a list of all the errors that SSIS can produce and it should be with us by RTM in November. That should be a great help in debugging your packages I reckon. Read more here: http://forums.microsoft.com/msdn/ShowPost.aspx?PostID=106107 Thanks to Doug Laudenshlager for the info!
An answer from Doug to Frans van Bree's comment (below):
"It's my understanding that errors, warnings, and informational messages from Microsoft-provided Integration Services components are all included in the HResults class already documented (though without the hex codes) in the API reference, as mentioned in my previous post.
These will also be included, with hexadecimal codes, in the new topic that I mentioned."