fix: prevent exception that could happen when adding a book

Author: bayang

src/main/kotlin/io/github/bayang/jelu/service/BookService.kt

@@ -152,8 +152,10 @@ class BookService(
             // existing book used on UserBook already had an image, backup it
             if (!book.image.isNullOrBlank()) {
                 currentImage = File(properties.files.images, book.image)
-                backup = File(properties.files.images, "${book.image}.bak")
-                Files.move(currentImage.toPath(), backup.toPath())
+                if (currentImage.exists()) {
+                    backup = File(properties.files.images, "${book.image}.bak")
+                    Files.move(currentImage.toPath(), backup.toPath())
+                }
             }
             book.image = saveImages(file, book.title, book.id.toString(), userBook.book.image, properties.files.images)
             // we had a previous image and we saved a new one : delete the old one

src/main/kotlin/io/github/bayang/jelu/service/metadata/FetchMetadataService.kt

@@ -101,6 +101,7 @@ class FetchMetadataService(
                 }
                 if (targetCover.exists() && targetCover.length() > 0) {
                     parseOpf.image = targetCover.name
+                    logger.trace { "fetch metadata image ${targetCover.name}" }
                 }
                 return parseOpf
             } else {