A server-sent Error that the handshake ack path has already forwarded to
result_tx. Typed so the outer retry loop can tell a real server refusal
from a plain network failure: the server’s own message must stay the last
one the GUI sees (no synthetic error-network on top of it), and a fatal
kind means retrying can never succeed (CLOUDPUB-1464). A fatal refusal is
what run_client returns, so a caller can tell it from a broken client
(CLOUDPUB-1590).