fix: return 500 status code to Stacks node when event ingestion fails (…

…#648)

This PR makes sure we return a `500` status code error to the Stacks
node when chainhook failed to ingest an event correctly, so it can retry
indefinitely until it succeeds.

This prevents block gaps from occurring in the chainhook index.

Fixes #646

components/chainhook-sdk/src/indexer/stacks/blocks_pool.rs

@@ -110,7 +110,7 @@ impl StacksBlockPool {
                 ctx.try_log(|logger| {
                     slog::error!(logger, "unable to retrieve previous stacks fork")
                 });
-                return Ok(None);
+                return Err("unable to retrieve previous stacks fork".to_string());
             }
         };