tor-hsservice: Replace ReactorError with FatalError

The publisher now returns FatalErrors, so we don't need ReactorError anymore. Addresses a TODO HSS in publish/reactor.rs

I consider this a prerequisite to #1083 (closed): it's easier to unify the error types for the TODO HSS: Add latest-error. in status.rs if the publisher returns FatalError like the IPT manager and OnionService instead of its own error type).

Part of #1129 (closed) and #1098