Check and return error values in goptlib

Make goptlib check and return error values of functions, instead of silently ignoring them.

git.torproject.org/pluggable-transports/goptlib.git/pt.go:557:15        defer f.Close()
git.torproject.org/pluggable-transports/goptlib.git/pt.go:629:16        io.WriteString(h, "ExtORPort authentication server-to-client hash")
git.torproject.org/pluggable-transports/goptlib.git/pt.go:630:9 h.Write(clientNonce)
git.torproject.org/pluggable-transports/goptlib.git/pt.go:631:9 h.Write(serverNonce)
git.torproject.org/pluggable-transports/goptlib.git/pt.go:638:16        io.WriteString(h, "ExtORPort authentication client-to-server hash")
git.torproject.org/pluggable-transports/goptlib.git/pt.go:639:9 h.Write(clientNonce)
git.torproject.org/pluggable-transports/goptlib.git/pt.go:640:9 h.Write(serverNonce)
git.torproject.org/pluggable-transports/goptlib.git/pt.go:857:15        s.SetDeadline(time.Now().Add(5 * time.Second))
git.torproject.org/pluggable-transports/goptlib.git/pt.go:868:15        s.SetDeadline(time.Time{})