Fail with better error message in microbenchmarks (#16676)

dev/devicelab/lib/tasks/microbenchmarks.dart

diff --git a/dev/devicelab/lib/tasks/microbenchmarks.dart b/dev/devicelab/lib/tasks/microbenchmarks.dart
index 81dd71e..6def31a 100644
--- a/dev/devicelab/lib/tasks/microbenchmarks.dart
+++ b/dev/devicelab/lib/tasks/microbenchmarks.dart
@@ -127,7 +127,12 @@
       jsonStarted = false;
       processWasKilledIntentionally = true;
       process.kill(ProcessSignal.SIGINT); // flutter run doesn't quit automatically
-      completer.complete(json.decode(jsonBuf.toString()));
+      final String jsonOutput = jsonBuf.toString();
+      try {
+        completer.complete(json.decode(jsonOutput));
+      } catch (ex) {
+        completer.completeError('Decoding JSON failed ($ex). JSON string was: $jsonOutput');
+      }
       return;
     }