minor fix

Author: kirk0830

tools/molden/molden.py

@@ -905,7 +905,8 @@ def moldengen(folder: str, ndigits=3, ngto=7, rel_r=2, fmolden="ABACUS.molden"):
     import numpy as np
 
     files = os.listdir(folder)
-    assert ("STRU" in files) and ("INPUT" in files) and ("KPT" in files)
+    assert ("STRU" in files) and ("INPUT" in files) and ("KPT" in files),\
+        "STRU, INPUT, KPT files are required in the folder."
     cwd = os.path.abspath(os.getcwd())
     os.chdir(folder)
     ####################