Retry downloading traefik binary if it fails

[sggpls]

  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100  3557  100  3557    0     0  10649      0 --:--:-- --:--:-- --:--:-- 10617
Checking if TLJH is already installed...
Setting up hub environment
Installed python & virtual environment
Set up hub virtual environment
Setting up TLJH installer...
Setup tljh package
Starting TLJH installer...
Setting up admin users
Granting passwordless sudo to JupyterHub admins...
Setting up user environment...
Downloading & setting up user environment...
Setting up JupyterHub...
Downloading traefik 1.7.5...
Traceback (most recent call last):
  File "/usr/lib/python3.6/runpy.py", line 193, in _run_module_as_main
    "__main__", mod_spec)
  File "/usr/lib/python3.6/runpy.py", line 85, in _run_code
    exec(code, run_globals)
  File "/opt/tljh/hub/lib/python3.6/site-packages/tljh/installer.py", line 455, in <module>
    main()
  File "/opt/tljh/hub/lib/python3.6/site-packages/tljh/installer.py", line 442, in main
    ensure_jupyterhub_package(HUB_ENV_PREFIX)
  File "/opt/tljh/hub/lib/python3.6/site-packages/tljh/installer.py", line 200, in ensure_jupyterhub_package
    traefik.ensure_traefik_binary(prefix)
  File "/opt/tljh/hub/lib/python3.6/site-packages/tljh/traefik.py", line 50, in ensure_traefik_binary
    urlretrieve(traefik_url, traefik_bin)
  File "/usr/lib/python3.6/urllib/request.py", line 277, in urlretrieve
    block = fp.read(bs)
  File "/usr/lib/python3.6/http/client.py", line 449, in read
    n = self.readinto(b)
  File "/usr/lib/python3.6/http/client.py", line 493, in readinto
    n = self.fp.readinto(b)
  File "/usr/lib/python3.6/socket.py", line 586, in readinto
    return self._sock.recv_into(b)
  File "/usr/lib/python3.6/ssl.py", line 1012, in recv_into
    return self.read(nbytes, buffer)
  File "/usr/lib/python3.6/ssl.py", line 874, in read
    return self._sslobj.read(len, buffer)
  File "/usr/lib/python3.6/ssl.py", line 631, in read
    v = self._sslobj.read(len, buffer)
ConnectionResetError: [Errno 104] Connection reset by peer```

[yuvipanda]

We should try make this retry. Can you try running the installer again? That should just work...