Intermittent URLError: <urlopen error [Errno 60] Operation timed out>

[upsuper]

See for example http://build.servo.org/builders/mac-rel-wpt2/builds/6388

Looking for rustc at /Users/servo/.servo/rust/2017-10-05-alt/rustc-nightly-x86_64-apple-darwin/rustc/bin/rustc
Error running mach:

    ['build', '--release']

The error occurred in code that was called by the mach command. This is either
a bug in the called code itself or in the way that mach is calling it.

You should consider filing a bug for this issue.

If filing a bug, please include the full output of mach, including this error
message.

The details of the failure are as follows:

URLError: <urlopen error [Errno 60] Operation timed out>

  File "/Users/servo/buildbot/slave/mac-rel-wpt2/build/python/servo/build_commands.py", line 233, in build
    self.ensure_bootstrapped(target=target)
  File "/Users/servo/buildbot/slave/mac-rel-wpt2/build/python/servo/command_base.py", line 605, in ensure_bootstrapped
    stable=self._use_stable_rust)
  File "/Users/servo/buildbot/slave/mac-rel-wpt2/build/python/_virtualenv/lib/python2.7/site-packages/mach/registrar.py", line 123, in dispatch
    return self._run_command_handler(handler, context=context, **kwargs)
  File "/Users/servo/buildbot/slave/mac-rel-wpt2/build/python/_virtualenv/lib/python2.7/site-packages/mach/registrar.py", line 90, in _run_command_handler
    result = fn(**kwargs)
  File "/Users/servo/buildbot/slave/mac-rel-wpt2/build/python/servo/bootstrap_commands.py", line 94, in bootstrap_rustc
    nightly_commit_hash = toml.load(urllib2.urlopen(channel))["pkg"]["rustc"]["git_commit_hash"]
  File "/usr/local/Cellar/python/2.7.12/Frameworks/Python.framework/Versions/2.7/lib/python2.7/urllib2.py", line 154, in urlopen
    return opener.open(url, data, timeout)
  File "/usr/local/Cellar/python/2.7.12/Frameworks/Python.framework/Versions/2.7/lib/python2.7/urllib2.py", line 429, in open
    response = self._open(req, data)
  File "/usr/local/Cellar/python/2.7.12/Frameworks/Python.framework/Versions/2.7/lib/python2.7/urllib2.py", line 447, in _open
    '_open', req)
  File "/usr/local/Cellar/python/2.7.12/Frameworks/Python.framework/Versions/2.7/lib/python2.7/urllib2.py", line 407, in _call_chain
    result = func(*args)
  File "/usr/local/Cellar/python/2.7.12/Frameworks/Python.framework/Versions/2.7/lib/python2.7/urllib2.py", line 1241, in https_open
    context=self._context)
  File "/usr/local/Cellar/python/2.7.12/Frameworks/Python.framework/Versions/2.7/lib/python2.7/urllib2.py", line 1198, in do_open
    raise URLError(err)