Better errors around networking not enabled

[aronchick]

When you run a job, networking can be wrong in two ways:

• The node does not have JobAdmissionControl.AcceptNetworkedJobs set to true.
• The job does not have network set to full or http

TBH, I'm not sure we need both any more. But in either case, this is not a particularly helpful error message.

Execution e-0d59d223:
error: Failed to fetch: `https://pypi.org/simple/pyyaml/`
  Caused by: Could not connect, are you offline?
  Caused by: Request failed after 3 retries
  Caused by: error sending request for url (https://pypi.org/simple/pyyaml/)
  Caused by: client error (Connect)
  Caused by: dns error: failed to lookup address information: Try again
  Caused by: failed to lookup address information: Try again

Any ideas how we can improve?

[linear]

ENG-633 Better errors around networking not enabled