Adding a unique error when no authentication method has been passed in

yet.

Since ServerAuthError returns all errors collected during the
userAuthLoop, it will always include the generic error "no auth passed
yet" (assuming the connection fails).

Change-Id: I5f6c67f3f0762b023618178d4028600d2b6c9253
Reviewed-on: https://go-review.googlesource.com/92737
Reviewed-by: Han-Wen Nienhuys <hanwen@google.com>
Run-TryBot: Han-Wen Nienhuys <hanwen@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

ssh/server.go

@@ -309,6 +309,14 @@ func (l ServerAuthError) Error() string {
 	return "[" + strings.Join(errs, ", ") + "]"
 }
 
+// NoAuthError is the unique error that is returned if no authentication method
+// has been passed yet
+type NoAuthError struct{}
+
+func (e *NoAuthError) Error() string {
+	return "no auth passed yet"
+}
+
 func (s *connection) serverAuthenticate(config *ServerConfig) (*Permissions, error) {
 	sessionID := s.transport.getSessionID()
 	var cache pubKeyCache
@@ -363,7 +371,7 @@ userAuthLoop:
 		}
 
 		perms = nil
-		authErr := errors.New("no auth passed yet")
+		authErr := error(&NoAuthError{})
 
 		switch userAuthReq.Method {
 		case "none":