Emit CocoaPods output if pod install fails (#11018)

In the case where the CocoaPods 'pod install' step fails, emit its
stdout to provide any potentially-useful error messages to the user.

packages/flutter_tools/lib/src/ios/mac.dart

@@ -434,16 +434,15 @@ Future<Null> _runPodInstall(Directory bundle, String engineDirectory) async {
       );
       return;
     }
-    try {
     final Status status = logger.startProgress('Running pod install...', expectSlowOperation: true);
-      await runCheckedAsync(
+    final ProcessResult result = await processManager.run(
         <String>['pod', 'install'],
         workingDirectory: bundle.path,
         environment: <String, String>{'FLUTTER_FRAMEWORK_DIR': engineDirectory},
     );
     status.stop();
-    } catch (e) {
-      throwToolExit('Error running pod install: $e');
+    if (result.exitCode != 0) {
+      throwToolExit('Error running pod install:\n${result.stdout}');
     }
   }
 }