stage: make the unmarshalling error more verbose

Prior this commit there was no information in the log about the unrecognized
stage. This commit adds the stage name to the unmarshalling error.

internal/osbuild/stage.go

@@ -2,7 +2,7 @@ package osbuild
 
 import (
 	"encoding/json"
-	"errors"
+	"fmt"
 )
 
 // A Stage transforms a filesystem tree.
@@ -67,7 +67,7 @@ func (stage *Stage) UnmarshalJSON(data []byte) error {
 	case "org.osbuild.script":
 		options = new(ScriptStageOptions)
 	default:
-		return errors.New("unexpected stage name")
+		return fmt.Errorf("unexpected stage name: %s", rawStage.Name)
 	}
 	err = json.Unmarshal(rawStage.Options, options)
 	if err != nil {