Misleading error message ("no valid primary key self-signature") when key has expired

This has been reported before (#304, #745, #1270) but it seems like the UX aspect was not considered. When trying to upload an expired key, the error message is unhelpful and sends the user on a wild-goose chase of trying to verify the signature. Ideally, the message would be more specific and different for each error scenario, but at least the message should be rephrased to include the possibility of expiration.

Search the client for the exact message “no valid primary key self-signature” and trace the expired-key upload path. Confirm how expiration is detected and update the user-facing wording so it mentions expiration; verify the resulting message for expired-key uploads and the other error scenarios covered by the existing flow.

Written by the indexing model from the issue text.