Pinpointe has a special custom field/variable that can be used to display error to your contacts when they encountered errors while using your "Web-Forms." For internal error pages that are stored in Pinpointe internally, you can use the custom field %%GLOBAL_Errors%% in order to display these error messages to your contact...
For external error pages where you "use existing URL" for your error pages, Pinpointe passes a special variable in the URL called "Errors."
An example of an external error page code:
<html> <head><title>Error has occurred</title></head>
The following error has been returned: <?php print $_GET['Errors']; ?>