pageserver: cleaner error handling if client sends sharded request to wrong pageserver

[jcsp]

In #5980:

            let timeline = match self
                .get_active_tenant_timeline(
                    timeline.tenant_shard_id.tenant_id,
                    timeline.timeline_id,
                    ShardSelector::Page(key),
                )
                .await
            {
                Ok(t) => t,
                Err(GetActiveTimelineError::Tenant(GetActiveTenantError::NotFound(_))) => {
                    // We already know this tenant exists in general, because we resolved it at
                    // start of connection.  Getting a NotFound here indicates that the shard containing
                    // the requested page is not present on this node.

                    // TODO: this should be some kind of structured error that the client will understand,
                    // so that it can block until its config is updated: this error is expected in the case
                    // that the Tenant's shards' placements are being updated and the client hasn't been
                    // informed yet.
                    return Err(anyhow::anyhow!("Request routed to wrong shard"));
                }
                Err(e) => return Err(e.into()),
   1         };

[jcsp]

https://github.com/neondatabase/neon/pull/6262/commits