GoFileEmpty: no retries for content that exists but is empty; add tools/debug_gofile.py for raw content-API inspection

dlphelper/gofile.py

@@ -60,6 +60,13 @@ class GoFileNotFound(GoFileError):
     """
 
 
+class GoFileEmpty(GoFileError):
+    """The content exists but contains no files (e.g. emptied folder).
+
+    Stable state like a dead link: no retry.
+    """
+
+
 class GoFileAuthError(GoFileError):
     """The account token was rejected; the client can re-auth once."""