Improve print-reference in logical blocks in src/conditions.lisp

* src/conditions.lisp (print-reference): use :fill-style conditional
  newlines at suitable positions
On SBCL, improve print-object reference-condition in logical blocks in src/conditions.lisp

* src/conditions.lisp (print-object :after reference-condition): on
  SBCL, act differently depending on whether the stream is a pretty
  printing stream
* test/conditions.lisp (define-condition-suite reference-error): adapted
  to changes
  (define-condition-suite mock-error/reference-condition): likewise
  (test reference-condition.print/logical-block): new test; test
  printing `reference-condition' within a logical block