fix: propagate failed img pull error to response line (#1302)

## Description:
Prior to this change, errors causing failed image pulls were not being
shown to the user.

In failed image pulls, user would see is `Failed fetching the required
image '%v', make sure that the image exists and is public"` - even if
that image was public and existed.

This PR propagates the image pull error all the way to the user to help
them better debug the failed image pull.

## Is this change user facing?
YES

core/server/api_container/server/startosis_engine/startosis_validator/docker_images_validator.go

@@ -63,7 +63,8 @@ func fetchImageFromBackend(ctx context.Context, wg *sync.WaitGroup, imageCurrent
 	err := (*backend).FetchImage(ctx, image)
 	if err != nil {
 		logrus.Warnf("Container image '%s' download failed. Error was: '%s'", image, err.Error())
-		pullErrors <- startosis_errors.NewValidationError("Failed fetching the required image '%v', make sure that the image exists and is public", image)
+		pullErrors <- startosis_errors.WrapWithValidationError(err, "Failed fetching the required image '%v'.", image)
+		return
 	}
 	logrus.Debugf("Container image '%s' successfully downloaded", image)
 }