Add Logging to Mock Repo API Server (#49409)

While we log exception in the handler, we may still miss exceptions
hgiher up the execution chain. This adds logging of exceptions to all
operations on the IO loop including connection establishment.

Relates #49401

test/framework/src/main/java/org/elasticsearch/repositories/blobstore/ESMockAPIBasedRepositoryIntegTestCase.java

@@ -22,6 +22,7 @@
 import com.sun.net.httpserver.HttpHandler;
 import com.sun.net.httpserver.HttpServer;
 import org.apache.http.HttpStatus;
+import org.apache.logging.log4j.LogManager;
 import org.apache.logging.log4j.Logger;
 import org.apache.logging.log4j.message.ParameterizedMessage;
 import org.elasticsearch.action.admin.indices.forcemerge.ForceMergeResponse;
@@ -72,9 +73,19 @@ protected interface BlobStoreHttpHandler extends HttpHandler {
     private static HttpServer httpServer;
     private Map<String, HttpHandler> handlers;
 
+    private static final Logger log = LogManager.getLogger();
+
     @BeforeClass
     public static void startHttpServer() throws Exception {
         httpServer = MockHttpServer.createHttp(new InetSocketAddress(InetAddress.getLoopbackAddress(), 0), 0);
+        httpServer.setExecutor(r -> {
+            try {
+                r.run();
+            } catch (Throwable t) {
+                log.error("Error in execution on mock http server IO thread", t);
+                throw t;
+            }
+        });
         httpServer.start();
     }