refuse to manual-collate a nonexistent path

dlibrary.py

@@ -208,6 +208,12 @@ def manual_collate(args):
     if collation_dir.exists() and len(list(collation_dir.iterdir())) > 0:
         print(f'Collation directory already exists!')
         return
+
+    nonexistent = [path for path in args.paths if not path.exists()]
+    if len(nonexistent) > 0:
+        print(f'Nonexistent paths: {nonexistent}')
+        return
+
     collation_dir.mkdir(parents=True, exist_ok=True)
 
     index = 0