ssh: return ServerAuthError after too many auth failures

if a client is disconnected due to too many authentication attempts
we should return a ServerAuthError instead of a generic error.

Some users check the error returned by NewServerConn to determine
whether or not a client attempted to authenticate.

Fixes golang/go#69191

Change-Id: If68fcecdefd6c810fe9df8256b1216e320d8a916
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/566398
Reviewed-by: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Tim King <taking@google.com>
Auto-Submit: Nicola Murino <nicola.murino@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>

ssh/server.go

@@ -510,8 +510,8 @@ userAuthLoop:
 			if err := s.transport.writePacket(Marshal(discMsg)); err != nil {
 				return nil, err
 			}
-
-			return nil, discMsg
+			authErrs = append(authErrs, discMsg)
+			return nil, &ServerAuthError{Errors: authErrs}
 		}
 
 		var userAuthReq userAuthRequestMsg