control master logging

Signed-off-by: Kimmo Lehto <[email protected]>

openssh.go

@@ -1,6 +1,7 @@
 package rig
 
 import (
+	"bytes"
 	"context"
 	"errors"
 	"fmt"
@@ -208,15 +209,16 @@ func (c *OpenSSH) Connect() error {
 	args = append(args, c.args()...)
 
 	cmd := goexec.Command("ssh", args...)
+	errBuf := bytes.NewBuffer(nil)
 	cmd.Stdout = nil
-	cmd.Stderr = nil
+	cmd.Stderr = errBuf
 	cmd.Stdin = nil
 
 	log.Debugf("%s: starting ssh control master", c)
 	err := cmd.Run()
 	if err != nil {
 		c.isConnected = false
-		return fmt.Errorf("failed to start ssh multiplexing control master: %w", err)
+		return fmt.Errorf("failed to start ssh multiplexing control master: %w (%s)", err, errBuf.String())
 	}
 
 	c.isConnected = true