Add logging if FlutterDartProject fails to load the application kernel snapshot (#6257)

To make "Engine run configuration was invalid." failures (e.g.
https://github.com/flutter/flutter/issues/20777) a bit easier to
diagnose, add some more logging to `FlutterDartProject` to verify
that snapshot assets were properly bundled.

shell/platform/darwin/ios/framework/Source/FlutterDartProject.mm

@@ -98,21 +98,22 @@ static blink::Settings DefaultSettingsForProcess(NSBundle* bundle = nil) {
     NSString* assetsName = [FlutterDartProject flutterAssetsName:bundle];
     NSString* assetsPath = [mainBundle pathForResource:assetsName ofType:@""];
 
-    if (assetsPath.length > 0) {
+    if (assetsPath.length == 0) {
+      NSLog(@"Failed to find assets path for \"%@\"", assetsName);
+    } else {
       settings.assets_path = assetsPath.UTF8String;
 
+      // Check if there is an application kernel snapshot in the assets directory we could
+      // potentially use.  Looking for the snapshot makes sense only if we have a VM that can use
+      // it.
       if (!blink::DartVM::IsRunningPrecompiledCode()) {
-        // Looking for the various script and kernel snapshot buffers only makes sense if we have a
-        // VM that can use these buffers.
-        {
-          // Check if there is an application kernel snapshot in the assets directory we could
-          // potentially use.
-          NSURL* applicationKernelSnapshotURL =
-              [NSURL URLWithString:@(kApplicationKernelSnapshotFileName)
-                     relativeToURL:[NSURL fileURLWithPath:assetsPath]];
-          if ([[NSFileManager defaultManager] fileExistsAtPath:applicationKernelSnapshotURL.path]) {
-            settings.application_kernel_asset = applicationKernelSnapshotURL.path.UTF8String;
-          }
+        NSURL* applicationKernelSnapshotURL =
+            [NSURL URLWithString:@(kApplicationKernelSnapshotFileName)
+                   relativeToURL:[NSURL fileURLWithPath:assetsPath]];
+        if ([[NSFileManager defaultManager] fileExistsAtPath:applicationKernelSnapshotURL.path]) {
+          settings.application_kernel_asset = applicationKernelSnapshotURL.path.UTF8String;
+        } else {
+          NSLog(@"Failed to find snapshot: %@", applicationKernelSnapshotURL.path);
         }
       }
     }