make friendlier message warning user that their creds may be wrong

helper/communicator/step_connect_ssh.go

@@ -215,6 +215,12 @@ func (s *StepConnectSSH) waitForSSH(state multistep.StateBag, ctx context.Contex
 			if strings.Contains(err.Error(), "authenticate") {
 				log.Printf(
 					"[DEBUG] Detected authentication error. Increasing handshake attempts.")
+				err = fmt.Errorf("Packer experienced an authentication error "+
+					"when trying to connect via SSH. This can happen if your "+
+					"username/password are wrong. You may want to double-check"+
+					" your credentials as part of your debugging process. "+
+					"original error: %s",
+					err)
 				handshakeAttempts += 1
 			}