We would like to extend the read RPC to return an optional, second value
which indicates an exception - seastar type-erases exception on the RPC
handler boundary and we need to differentiate rate_limit_exception from
others. However, it may happen that a replica with an up-to-date version
of Scylla tries to return an exception in this way to a coordinator with
an old version and the coordinator will drop the error, thinking that
the request succeeded.
In order to protect from that, we introduce the
`TYPED_ERROR_IN_READ_RPC` feature. Only after it is enabled replicas
will start returning exceptions in the new way, and until then all
exceptions will be reported using seastar's type-erasure mechanism.
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