MEDIUM: proxy: Add a directive to reference an http-errors section in a proxy

MEDIUM: proxy: Add a directive to reference an http-errors section in a proxy

It is now possible to import in a proxy, fully or partially, error files
declared in an http-errors section. It may be done using the "errorfiles"
directive, followed by a name and optionally a list of status code. If there is
no status code specified, all error files of the http-errors section are
imported. Otherwise, only error files associated to the listed status code are
imported. For instance :

  http-errors my-errors
      errorfile 400 ...
      errorfile 403 ...
      errorfile 404 ...

  frontend frt
      errorfiles my-errors 403 404  # ==> error 400 not imported