json: switch backup and restore errors from string to struct types

This keeps backwards compatibility with the previous empty structs.
And maybe we'd want to put other fields into the inner struct later,
rather than the outer message.

internal/ui/restore/json.go

@@ -48,7 +48,7 @@ func (t *jsonPrinter) Update(p State, duration time.Duration) {
 func (t *jsonPrinter) Error(item string, err error) error {
 	t.error(errorUpdate{
 		MessageType: "error",
-		Error:       err.Error(),
+		Error:       errorObject{err.Error()},
 		During:      "restore",
 		Item:        item,
 	})
@@ -113,11 +113,15 @@ type statusUpdate struct {
 	BytesSkipped   uint64  `json:"bytes_skipped,omitempty"`
 }
 
+type errorObject struct {
+	Message string `json:"message"`
+}
+
 type errorUpdate struct {
-	MessageType string `json:"message_type"` // "error"
-	Error       string `json:"error"`
-	During      string `json:"during"`
-	Item        string `json:"item"`
+	MessageType string      `json:"message_type"` // "error"
+	Error       errorObject `json:"error"`
+	During      string      `json:"during"`
+	Item        string      `json:"item"`
 }
 
 type verboseUpdate struct {