Retry IPFS request on Cloudflare 521 Web Server Down (#5687)

In the integration tests, some subgraphs will sync on fraction3, leading
to an error as they failed to sync on integer.

On some runs, fraction3 will also error leading to a successful test;
this happens when the subgraph fails to sync due to an HTTP 521 error
received when querying the testnet IPFS node. This is a
non-deterministic error, and the request can simply be retried.

graph/src/ipfs/error.rs

@@ -87,11 +87,14 @@ impl RequestError {
             return true;
         };
 
+        const CLOUDFLARE_WEB_SERVER_DOWN: u16 = 521;
+
         [
             StatusCode::TOO_MANY_REQUESTS,
             StatusCode::INTERNAL_SERVER_ERROR,
             StatusCode::BAD_GATEWAY,
             StatusCode::SERVICE_UNAVAILABLE,
+            StatusCode::from_u16(CLOUDFLARE_WEB_SERVER_DOWN).unwrap(),
         ]
         .into_iter()
         .any(|x| status == x)