afws_client: improve error message when output already exists

artiq/frontend/afws_client.py

@@ -132,9 +132,13 @@ def main():
             try:
                 os.mkdir(args.directory)
             except FileExistsError:
+                try:
                     if any(os.scandir(args.directory)):
                         print("Output directory already exists and is not empty. Please remove it and try again.")
                         sys.exit(1)
+                except NotADirectoryError:
+                    print("A file with the same name as the output directory already exists. Please remove it and try again.")
+                    sys.exit(1)
             rev = args.rev
             if rev is None:
                 rev = get_artiq_rev()