Reraise Excon errors as K8s::Errors

[kke]

Fixes #162

User should not care / know about transport backend.

• Excon::Error::Certificate / Excon::Errors::CertificateError is raised as K8s::Error::SSL
• Excon::Error::Socket / Excon::Errors::SocketError is raised as K8s::Error::Socket
• Any unspecified (that were not already catched as K8s::Error::API, K8s::Error::HTTP_STATUS_ERRORS) Excon errors are reraised as K8s::Error::Transport.
• rescuing K8s::Error::Transport will also rescue from ::SSL and ::Socket as they are inherited from ::Transport.