You should be able to add error messages to the page through the bindings interface in dreamweaver.
Go to the bindings tab and click the plus button. Then choose eCart->Checkout->Local checkout results.
Then choose the error result you would like to see more information about... it appears you are using paypal direct pay transaction, so choose that one and click ok.
Then you will be able to drag and drop more information about why the failure occurred such as an error message.