SeriesServiceImpl.addImageToSeries(): fix error message in asertion.

No functional changes.

Author: php-coder

src/main/java/ru/mystamps/web/service/SeriesServiceImpl.java

@@ -128,7 +128,7 @@ public Integer add(AddSeriesDto dto, User user, boolean userCanAddComments) {
 	@PreAuthorize("hasAuthority('ADD_IMAGES_TO_SERIES')")
 	public void addImageToSeries(AddImageDto dto, Series series) {
 		Validate.isTrue(dto != null, "DTO must be non null");
-		Validate.isTrue(series != null, "DTO must be non null");
+		Validate.isTrue(series != null, "Series must be non null");
 
 		Image image = imageService.save(dto.getImage());