Fix crash when stylesheet is not available

All browsers ignore unavailable stylesheets and print a message to the console to this effect. Instead, WeasyPrint crashes.

Example: http://www.w3.org/Style/CSS/Test/Print/1.0/20070802/t1302-page-01-b.htm

    Traceback (most recent call last):
      File "/home/ariofrio/Repositories/WeasyPrint/env/bin/weasyprint", line 8, in <module>
        load_entry_point('WeasyPrint==0.19.1', 'console_scripts', 'weasyprint')()
      File "/home/ariofrio/Repositories/WeasyPrint/weasyprint/__main__.py", line 141, in main
        getattr(html, 'write_' + format_)(output, **kwargs)
      File "/home/ariofrio/Repositories/WeasyPrint/weasyprint/__init__.py", line 159, in write_pdf
        return self.render(stylesheets).write_pdf(target, zoom)
      File "/home/ariofrio/Repositories/WeasyPrint/weasyprint/__init__.py", line 131, in render
        return Document._render(self, stylesheets, enable_hinting)
      File "/home/ariofrio/Repositories/WeasyPrint/weasyprint/document.py", line 253, in _render
        for css in stylesheets or []])
      File "/home/ariofrio/Repositories/WeasyPrint/weasyprint/css/__init__.py", line 431, in get_all_computed_styles
        element_tree, device_media_type, url_fetcher))
      File "/home/ariofrio/Repositories/WeasyPrint/weasyprint/css/__init__.py", line 189, in find_stylesheets
        _check_mime_type=True, media_type=device_media_type)
      File "/home/ariofrio/Repositories/WeasyPrint/weasyprint/__init__.py", line 219, in __init__
        check_css_mime_type=_check_mime_type,)
      File "/home/ariofrio/Repositories/WeasyPrint/weasyprint/__init__.py", line 271, in _select_source
        result = url_fetcher(url)
      File "/home/ariofrio/Repositories/WeasyPrint/weasyprint/urls.py", line 281, in wrapped_fetcher
        result = url_fetcher(url)
      File "/home/ariofrio/Repositories/WeasyPrint/weasyprint/urls.py", line 281, in wrapped_fetcher
        result = url_fetcher(url)
      File "/home/ariofrio/Repositories/WeasyPrint/weasyprint/urls.py", line 262, in default_url_fetcher
        url, headers={'User-Agent': VERSION_STRING}))
      File "/home/ariofrio/Repositories/WeasyPrint/weasyprint/compat.py", line 80, in urlopen_contenttype
        result = urlopen(url)
      File "/usr/lib64/python2.7/urllib2.py", line 126, in urlopen
        return _opener.open(url, data, timeout)
      File "/usr/lib64/python2.7/urllib2.py", line 406, in open
        response = meth(req, response)
      File "/usr/lib64/python2.7/urllib2.py", line 519, in http_response
        'http', request, response, code, msg, hdrs)
      File "/usr/lib64/python2.7/urllib2.py", line 444, in error
        return self._call_chain(*args)
      File "/usr/lib64/python2.7/urllib2.py", line 378, in _call_chain
        result = func(*args)
      File "/usr/lib64/python2.7/urllib2.py", line 527, in http_error_default
        raise HTTPError(req.get_full_url(), code, msg, hdrs, fp)
    urllib2.HTTPError: HTTP Error 404: Not Found