code: def render_error(request, exc):
      '''Default renderer for errors.'''
      cfg = request.get('pulsar.cfg')
      debug = cfg.debug if cfg else False
      response = request.response
      if not response.content_type:
          content_type = request.get('default.content_type')
          response.content_type = request.content_types.best_match(
              as_tuple(content_type or DEFAULT_RESPONSE_CONTENT_TYPES)
          )
      content_type = None
      if response.content_type:
          content_type = response.content_type.split(';')[0]
      is_html = content_type == 'text/html'
  
      if debug:
          msg = render_error_debug(request, exc, is_html)
      else:
          msg = escape(error_messages.get(response.status_code) or exc)
          if is_html:
              msg = textwrap.dedent().format({"reason": response.status, "msg": msg,
                           "version": request.environ['SERVER_SOFTWARE']})
      #
      if content_type == 'text/html':
          doc = HtmlDocument(title=response.status)
          doc.head.embedded_css.append(error_css)
          doc.body.append(Html('div', msg, cn='pulsar-error'))
          return doc.to_bytes(request)
      elif content_type in JSON_CONTENT_TYPES:
          return json.dumps({'status': response.status_code,
                             'message': msg})
      else:
          return '\n'.join(msg) if isinstance(msg, (list, tuple)) else msg
docs: Default renderer for errors.