#11 add built-in anchor for error

[Andrew W Crapo]

getScriptFailureReturnValue returns the value of apvf:returnOnFailure, if assigned. The addition of a built-in anchor, e.g., "errorMessage", would allow the actual error description to be assigned as the return value. While this is done in the UI client, handling there does not allow the PO to be displayed normally and other processing to continue normally.