Add documentation, improve errors, and add changelog entry
* Document the limitation on the implementation of the support
of Ogmios as a target that limits the throughput to one request
in flight per round trip (Ogmios supports pipelining by JSON-RPC id,
but we are not supporting it for now.
* Document that only submission goes through Ogmios:
protocol-parameter and era queries as well as protocol startup still
require the local node socket and config file.
* Route per-transaction rejections through the benchmark tracer instead
of putStrLn, so they reach the trace stream like every other
submission event instead of interleaving arbitrarily with it. The
failure detail payload reported by Ogmios ('error.data'), which
carries the actual ledger failure and was previously discarded, is
included in the message along with the error code.
* Add `ogmiosUrl` to the README's connection-settings table plus a
'Submitting through Ogmios' section, add a changelog entry for the
feature (including the clean-exit behavior change for scripts that
never start the benchmark machinery), and bump the package version
to 2.17.
