_send_http_request collapses every exception path into a {"error": ...}
dict, so the if "error" in response branch inside send() is the only
place where send failures surface. Logging-only there meant the
ChannelManager retry mechanism never fired. Raise RuntimeError so the
base-class retry path is exercised; the outer try/except already
re-raises into the caller.
Addresses review comment on PR #3852.
2d81cc0ae1