--trace-startup: non-zero exit code when fails; enable in iOS runtime (#5309)

packages/flutter_tools/lib/src/run.dart

diff --git a/packages/flutter_tools/lib/src/run.dart b/packages/flutter_tools/lib/src/run.dart
index 1b4f2e8..ad1d3bf 100644
--- a/packages/flutter_tools/lib/src/run.dart
+++ b/packages/flutter_tools/lib/src/run.dart
@@ -188,7 +188,12 @@
 
     if (serviceProtocol != null && traceStartup) {
       printStatus('Downloading startup trace info...');
-      await downloadStartupTrace(serviceProtocol);
+      try {
+        await downloadStartupTrace(serviceProtocol);
+      } catch(error) {
+        printError(error);
+        return 2;
+      }
       appFinished();
     } else {
       setupTerminal();