While fetching CDC generations, various exceptions can occur. They
are divided into "fatal" and "nonfatal", where "fatal" ones prevent
retrying of the fetch operation.
This patch makes `read_failure_exception` "non-fatal", because such
error may appear during restart. In general this type of error can
mean a few different things (e.g. an error code in a response from
replica, but also a broken connection) so retrying seems reasonable.
Fixes#6804