improve cbz export

lib/pages/local_comics_page.dart

@@ -404,8 +404,9 @@ class _LocalComicsPageState extends State<LocalComicsPage> {
               var file = await CBZ.export(c);
               await saveFile(filename: file.name, file: file);
               await file.delete();
-            } catch (e) {
+            } catch (e, s) {
               context.showMessage(message: e.toString());
+              Log.error("CBZ Export", e, s);
             }
             controller.close();
           }),

lib/utils/cbz.dart

@@ -208,6 +208,7 @@ abstract class CBZ {
       ),
     );
     var cbz = File(FilePath.join(App.cachePath, sanitizeFileName('${comic.title}.cbz')));
+    if (cbz.existsSync()) cbz.deleteSync();
     await _compress(cache.path, cbz.path);
     cache.deleteSync(recursive: true);
     return cbz;