fix: report a locked folder as FolderLockedException

Fetching a locked folder before its password is sent returns 200 with
only its listing fields, without metadata, sub_folders and path, so
FoldersApi.retrieve failed with a pydantic ValidationError. The DOI
route fetches the folder before it can see the lock, so a
password-protected DOI folder could not be opened even with its
password.

- Add FolderLockedException, a subclass of UnauthorizedException, and
  raise it from retrieve on that short record and from the lock checks.
- On a DOI folder, send the password and fetch it again when the first
  fetch finds it locked.

mdrsclient/exceptions.py

@@ -28,6 +28,12 @@ class UnauthorizedException(MDRSException):
     pass
 
 
+class FolderLockedException(UnauthorizedException):
+    """Thrown when a folder cannot be opened without its password"""
+
+    pass
+
+
 class TokenExpiredException(UnauthorizedException):
     """Thrown when the access token was still valid when sent but had expired on arrival"""