Better Logging GCS Blobstore Mock (#50102)

* Better Logging GCS Blobstore Mock

Two things:
1. We should just throw a descriptive assertion error and figure out why we're not reading a multi-part instead of
returning a `400` and failing the tests that way here since we can't reproduce these 400s locally.
2. We were missing logging the exception on a cleanup delete failure that coincides with the `400` issue in tests.

Relates #49429

server/src/main/java/org/elasticsearch/repositories/blobstore/BlobStoreRepository.java

@@ -1186,7 +1186,7 @@ public abstract class BlobStoreRepository extends AbstractLifecycleComponent imp
                             try {
                                 blobContainer().deleteBlobsIgnoringIfNotExists(oldIndexN);
                             } catch (IOException e) {
-                                logger.warn("Failed to clean up old index blobs {}", oldIndexN);
+                                logger.warn(() -> new ParameterizedMessage("Failed to clean up old index blobs {}", oldIndexN), e);
                             }
                         }));
                     }

test/fixtures/gcs-fixture/src/main/java/fixture/gcs/GoogleCloudStorageHttpHandler.java

@@ -189,7 +189,8 @@ public class GoogleCloudStorageHttpHandler implements HttpHandler {
                     exchange.sendResponseHeaders(RestStatus.OK.getStatus(), response.length);
                     exchange.getResponseBody().write(response);
                 } else {
-                    exchange.sendResponseHeaders(RestStatus.BAD_REQUEST.getStatus(), -1);
+                    throw new AssertionError("Could not read multi-part request to [" + request + "] with headers ["
+                        + new HashMap<>(exchange.getRequestHeaders()) + "]");
                 }
 
             } else if (Regex.simpleMatch("POST /upload/storage/v1/b/" + bucket + "/*uploadType=resumable*", request)) {