Return error with stderr value

To maintain existing behaviour when not capturing stdout and stderr

pkg/commands/os.go

@@ -115,11 +115,12 @@ func sanitisedCommandOutput(output []byte, err error) (string, error) {
 	outputString := string(output)
 	if err != nil {
 		// errors like 'exit status 1' are not very useful so we'll create an error
-		// from the combined output
-		if outputString == "" {
-			return "", WrapError(err)
+		// from stderr if we got an ExitError
+		exitError, ok := err.(*exec.ExitError)
+		if ok {
+			return outputString, errors.New(string(exitError.Stderr))
 		}
-		return outputString, errors.New(outputString)
+		return "", WrapError(err)
 	}
 	return outputString, nil
 }