[MINOR] cli: "show errors" should display "backend <NONE>" when backend was not used

[MINOR] cli: "show errors" should display "backend <NONE>" when backend was not used

It was disturbing to see a backend name associated with a bad request
when this "backend" was in fact the frontend. Instead, we now display
"backend <NONE>" if the "backend" has no backend capability :

> show errors

[25/Mar/2010:06:44:25.394] frontend fe (#1): invalid request
  src 127.0.0.1, session #0, backend <NONE> (#-1), server <NONE> (#-1)
  request length 45 bytes, error at position 0: