[New] Connection Error YARF exit code

[douglasdotc]

Description

This PR adds YARF custom exit code for platform connection error.

Resolved issues

https://warthogs.atlassian.net/browse/YARF-60
closes #86

Documentation

Tests

[p-gentili]

Thanks for addressing the issue, but IMHO we should take a different direction because we definitely don't want to deal with RF lifecycle (see comment below).

I think for once we should ask for permission rather than forgiveness, adding a simple check_connection function in each platform that we run before even starting the suite. We don't want this to be a breaking change, hence please make the base method a simple logger.warning("Not implemented at platform level"); return True.

[p-gentili]

We are still hiding something below, for instance in Vnc we have yield inside the try/except block, which makes it catch exceptions raised by the user, not the connection function. Now that we added the synchrounous connection test, which runs immediately before starting the test suite, I would recommend to revert those try/except blocks inside the libraries.

[p-gentili]

Thank you very much for all the changes, this is good to go!