I feel your pain - the cryptic errors messages from epubcheck make it unnecessarily tough to fix the validation problems. There is a list of several error messages on the epubcheck wiki, but it isn't a complete list:
http://code.google.com/p/epubcheck/wiki/Errors
Even better than a list of common error messages would be to fix the error messages themselves! Soon I'm hoping to have some time to dig into the epubcheck code and improve some of the error messages.