Bug 1669100 - Add more logging to benchmark script r=perftest-reviewers,sparky

Differential Revision: https://phabricator.services.mozilla.com/D126498

testing/raptor/browsertime/browsertime_benchmark.js

@@ -71,6 +71,9 @@ module.exports = async function(context, commands) {
         });
       }
       ret = true;
+    } else {
+      context.log.error("No data collected from benchmark.");
+      ret = false;
     }
   }
   context.log.info("Browsertime benchmark ended.");