agent: return error when cert auth fails

[zimbabao]

Sites without intermediate certs in chain
and other ssl error ends up with sockets.ssl
being null and that causes Type error in agent.

[dawsonc623]

@indutny It seems like this project has been abandoned, but my organization has been using it for the last couple of years. This issue is preventing us from a new release of one of our products. If the lint errors are fixed, could we get this merged and a patch release out?

If not, we can fork the project and fix it up ourselves, but I would rather continue to use and contribute to the original, open source version here if possible.

[indutny]

@dawsonc623 could you send me an email (it is my github profile). I'd be happy to discuss this!