The HTTPServerError class is accessible via the paste.httpexceptions module.
This is an error condition in which the server is presumed to be in-error. This is usually unexpected, and thus requires a traceback; ideally, opening a support ticket for the customer. Unless specialized, this is a '500 Internal Server Error'
'The server has either erred or is incapable of performing\r\nthe requested operation.\r\n'
This exception as a WSGI application
See the source for more information.