Currently error display in Salesforce on standard pages must be done either as plain text or as embedding HTML within the error itself. However, using HTML in the error messages is not fully supported throughout Salesforce and makes for difficult to parse errors when received via the Web Service API rather than the main UI.
If it were possible to specify an inline VisualForce Page or component to handle display of errors, this would allow developers to use nicely formatted errors, with additional functionality while separating the display of the error from the actual error itself. There are several reasons why HTML/VF errors would be useful:
Linking to documentation
Allow the VF component/page to record the error message in a logging object
Link to other records within salesforce
This could be done with some simple modification to the current addError() method:
sObject.addError( PageReference page, String errorText, Integer optionalWidth, Integer optionalHeight );
The the error is returned during a non-UI call, only the errorText is returned. Otherwise an inline VisualForce page is displayed instead of a plain-text or HTML error message.