Format string fixes in src/conditions.lisp

* src/conditions.lisp (print-object :after reference-condition t): use
  mandatory pretty-print newline for non-sbcl case
  (print-arguments): removed illegal @ modifier from ; directive
format-{control,arguments} in simple-progress-condition in src/conditions.lisp

* src/conditions.lisp (simple-progress-condition): added default
  initargs format-control and format-arguments
* test/conditions.lisp (test maybe-print-explanation.print): do not try
  to construct an instance without format-control and format-arguments
  initargs on on-sbcl