After feature levels are introduced to Signed References, and downgrade
attack protections are implemented, new errors are encountered when
reading Signed References that are in a downgraded state.
Such downgrade errors are recoverable by advancing history in a way that
restores the feature level that the history requires.
However, the implementation in `radicle-fetch` must be adjusted to not
abort fetching on downgraded histories, but rather inspect the error in
this case and decide whether continuing the fetching process would
actually help recover from the downgraded state.
The two cases are treated slightly differently for non-delegates and
delegates. Since delegates are crucial for evaluating repository
identity, also the fact that they cannot recover from a downgraded state
is logged at the "info" level.
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