Inform the control port with an HS_DESC failed event when the client is unable
to pick an HSDir. It's followed by an empty HS_DESC_CONTENT event. In order to
achieve that, some control port code had to be modified to accept a NULL HSDir
identity digest.
This commit also adds a trigger of a failed event when we are unable to
base64-decode the descriptor cookie.
Fixes#22042
Signed-off-by: David Goulet <dgoulet@torproject.org>
