Introduce _RpcClientError and _RpcInternalError as distinct subclasses
of _RpcError so the dispatcher can handle bad requests and server-side
faults differently — returning client errors verbatim and logging
internal faults with their cause before replying ERR_INTERNAL.
Wrap write_proposal and archive_proposal IO with _RpcInternalError
so OS failures surface through the typed path instead of the bare
Exception fallback. All existing raise _RpcError(...) call sites
converted to _RpcClientError.
Closes#253
didericis-claude