[vm/ffi] Add embedder API for resolving asset ids - fix

Stop swallowing error messages.

TEST=Tested in the local engine build in Flutter that uses the new API.

Change-Id: I179366b8d5a251bcb00b5fea722e440c2745a1c7
Cq-Include-Trybots: luci.dart.try:pkg-linux-debug-try,pkg-linux-release-arm64-try,pkg-linux-release-try,pkg-mac-release-try,pkg-mac-release-arm64-try,pkg-win-release-try,vm-ffi-android-debug-arm-try,vm-aot-linux-debug-x64-try,vm-linux-debug-x64-try,vm-win-debug-x64-try,vm-mac-debug-x64-try,vm-mac-debug-arm64-try
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/389740
Auto-Submit: Daco Harkes <[email protected]>
Commit-Queue: Martin Kustermann <[email protected]>
Reviewed-by: Martin Kustermann <[email protected]>

runtime/lib/ffi_dynamic_library.cc

@@ -452,7 +452,7 @@ intptr_t FfiResolveInternal(const String& asset,
 
   // Native assets resolution.
   void* asset_result = FfiResolveAsset(thread, asset, symbol, error);
-  if (asset_result != nullptr) {
+  if (asset_result != nullptr || *error != nullptr) {
     return reinterpret_cast<intptr_t>(asset_result);
   }