This avoids a privacy leak when the web.static= directory is configured
but doesn't exist (which is almost always, since we set `web.static =
public_html` in the default config file, but nothing automatically
creates it). The nevow.static.File class tries to os.stat() the
directory before doing anything else, which causes an exception, which
renders the traceback to the HTTP client as a 500 Internal Server Error,
and the traceback includes the full path of the missing public_html
directory, which reveals the node's basedir.
Plain twisted.web.static.File doesn't do this check, and a missing
web.static directory just results in a plain old 404.
Closes ticket:1720.
