In order to be able to re-connect on certain errors, we model
connection errors separately from parsing errors. We also change
the API of the whole module to no longer forward all errors to
the subscribers but instead, only update the subscribers with
either a latest rate or a permanent failure in case we exhausted
all our options to re-connect the websocket.
To model all of this properly, we introduce to sub-modules so that
each submodule can have their own `Error` type.
Resolves#297.
Instead of leaking the tokio::sync:⌚:Receiver type in our
return value, we create a newtype that implements the desired
interface. This allows us to get rid of the `RateService` structs
and instead implement `LatestRate` directly on top of this struct.
Given that `LatestRate` is only used within the event_loop module,
we move the definition of this type into there.
