Fix runner.js crash - basic check for LHR existence

lib/runner.js

@@ -147,6 +147,9 @@ async function _executeTest(url, options) {
 
     // Prepare JSON Summary
     const lhr = runnerResult.lhr;
+    if (!lhr) {
+        throw new Error('Lighthouse failed to generate a report (no LHR).');
+    }