(define-condition parsing-error (error)
  ((error :initarg :error :reader parsing-error.error)
   (when  :initarg :when  :reader parsing-error.when))
  (:report (lambda (condition stream)
             (format stream "~A~%~{~A~%~}"
                     (parsing-error.error condition)
                     (parsing-error.when condition)))))

(defvar *when*     '())

(defmacro with-parsing-error-wrapper (&body body)
  `(handler-bind ((error (lambda (err)
                           (error 'parsing-error :error err
                                                 :when (copy-seq *when*)))))
     ,@body))


(let ((*when* (cons (format nil "Parsing slot clause ~S" clause) *when*)))
   ...)