Beef up analysis server integration test messages.

pkg/analysis_server/benchmark/integration/driver.dart

Index: pkg/analysis_server/benchmark/integration/driver.dart
diff --git a/pkg/analysis_server/benchmark/integration/driver.dart b/pkg/analysis_server/benchmark/integration/driver.dart
index aac45a2514a842f94a0623fb95ff06e9079bafd1..052206e6b60b75994c20bd9e2bf2a9cab45ad2ce 100644
--- a/pkg/analysis_server/benchmark/integration/driver.dart
+++ b/pkg/analysis_server/benchmark/integration/driver.dart
@@ -134,7 +134,7 @@ class Driver extends IntegrationTestMixin {
       // doesn't exit, then forcibly terminate it.
       sendServerShutdown();
       await server.exitCode.timeout(timeout, onTimeout: () {
-        return server.kill();
+        return server.kill('server failed to exit');
       });
     }
     _resultsReady();