Also removes the expectation Batches published via it are sent in an ordered
fashion. That won't be true if the signing protocols complete out-of-order (as
possible when we are signing them in parallel).
This isn't necessary. Because we just log the error, we never match off of it,
we don't need any structure beyond String (or now Debug, which still gives us
a way to print the error). This is for the ergonomics of not having to
constantly write `.map_err(|e| format!("{e:?}"))`.
It's not only helpful (to easily check where Serai's view of the external
network is) but it's necessary in case of a non-trivial chain fork to determine
which blockchain Serai considers canonical.
