[shell-operator] add StderrError type for typed stderr error handling

Author: Fkuloff

pkg/executor/executor.go

@@ -35,11 +35,11 @@ func Run(cmd *exec.Cmd) error {
 // StderrError is returned by RunAndLogLines when a command fails and produces
 // output on stderr. Callers can use errors.As to access the raw stderr content.
 type StderrError struct {
-	Stderr string
+	Message string
 }
 
 func (e *StderrError) Error() string {
-	return fmt.Sprintf("stderr: %s", e.Stderr)
+	return e.Message
 }
 
 type Executor struct {
@@ -157,7 +157,7 @@ func (e *Executor) RunAndLogLines(ctx context.Context, logLabels map[string]stri
 	err := e.cmd.Run()
 	if err != nil {
 		if len(stdErr.Bytes()) > 0 {
-			return nil, &StderrError{Stderr: stdErr.String()}
+			return nil, &StderrError{Message: stdErr.String()}
 		}
 
 		return nil, fmt.Errorf("cmd run: %w", err)