The `BlockCache::truncate` default method is removed because the
`trait-variant` crate does not yet support default methods.

Instead of requiring `zcash_client_backend::data_api::chain::Error`, it
now works with any error type that `BlockSource::Error` can be converted
into. We require an explicit conversion function instead of using `From`
because in the specific case of `data_api::chain::Error` it would result
in conflicting `From` impls.

…ache trait

Co-authored-by: Jack Grigg <[email protected]>