Agreed to your points & want to echo the same that for the Errors/Exceptions which gets triggered from the Pega OOTB/Engine level, the Application Developers doesn't have any control nor can customize them. We should have a STANDARD to display the relevant UI Error Messages to the Users whenever any Exception happens.
To follow up on this request, there should also be a distinction between 'User Friendly' and 'Developer Friendly' messages. If the website is an external, public application (or even if it is used internally by front-line users), technical msgs do not provide a good end user experience. One option could be a user friendly message, with a collapsed section that contains more technical details.