when prettify is set to true, server.py should return error/ instead of error.html

[fedelibre]

I'm using cactus installed from current master via pip.
If config.json contains "prettify": true and I enter a wrong URL, cactus searches error.html instead of error/index.html. This is the error message:

Uncaught exception in write_error
Traceback (most recent call last):
  File "/home/fede/.virtualenvs/cactus/local/lib/python2.7/site-packages/tornado/web.py", line 976, in send_error
    self.write_error(status_code, **kwargs)
  File "/home/fede/.virtualenvs/cactus/src/cactus/cactus/server.py", line 61, in write_error
    return self.render("error.html")
  File "/home/fede/.virtualenvs/cactus/local/lib/python2.7/site-packages/tornado/web.py", line 704, in render
    html = self.render_string(template_name, **kwargs)
  File "/home/fede/.virtualenvs/cactus/local/lib/python2.7/site-packages/tornado/web.py", line 808, in render_string
    t = loader.load(template_name)
  File "/home/fede/.virtualenvs/cactus/local/lib/python2.7/site-packages/tornado/template.py", line 343, in load
    self.templates[name] = self._create_template(name)
  File "/home/fede/.virtualenvs/cactus/local/lib/python2.7/site-packages/tornado/template.py", line 370, in _create_template
    with open(path, "rb") as f:
IOError: [Errno 2] No such file or directory: u'/home/fede/web/cactus/.build/error.html'
404 GET /wrong/