Better error when failing HELO

[john8329]

I found that using smtp-relay.gmail.com while using the localhost LocalName makes the library return an obscure EOF error, while the packet tracer reveals the actual response:

220 smtp-relay.gmail.com ESMTP f10sm1a4899ejl.97 - gsmtp
EHLO localhost
421 4.7.0 Try again later, closing connection. (EHLO) f10sm1a4899ejl.97 - gsmtp
HELO localhost

Specifying a different LocalName fixes the issue, still it makes it hard to debug, it'd be nice to check the server response in smtp.go:64 and return the server error instead of just EOF.