Fail fast and suggest the user to run in --offline if no internet

[dliappis]

While working on #1467 we figured that the current mode of operation which a) validates internet connectivity by
attempting a successful connection to github.com and b) if not true, runs Rally in offline mode

is doing too much implicitly and might be difficult to troubleshoot if the intention is to run normally but there is a small github outage.

Instead an alternative would be that if all retries have failed, we should raise an error and advise the user to specify --offline, if that's what they intend.