No internet error message is bad

[konstin]

The error message when there is no internet connection is bad, both for pip compile and for pip sync:

$  pip compile scripts/requirements/jupyter.in
error: error sending request for url (https://pypi.org/simple/jupyter/): error trying to connect: dns error: failed to lookup address information: Temporary failure in name resolution
  Caused by: error trying to connect: dns error: failed to lookup address information: Temporary failure in name resolution
  Caused by: dns error: failed to lookup address information: Temporary failure in name resolution
  Caused by: failed to lookup address information: Temporary failure in name resolution

We should remove the duplication and instead of the arcane DNS error (If it's a temporary failure, when will it be over? And why is it complaining about DNS, the DNS entry of our index is correct?) we should hint the user that they have no internet connection.

[zanieb]

Agree ran into this on the plane.