### Preflight Checklist

- [X] I have read the [Contributing Guidelines](https://github.com/electron/electron/blob/master/CONTRIBUTING.md) for this project.
- [X] I agree to follow the [Code of Conduct](https://github.com/electron/electron/blob/master/CODE_OF_CONDUCT.md) that this project adheres to.
- [X] I have searched the [issue tracker](https://www.github.com/electron/electron/issues) for a feature request that matches the one I want to file, without success.

### Problem Description

We need to load a custom error page when the certificate is invalid but without "isMainFrame" property we don't know when to load the custom error page using webContents.loadURL()

### Proposed Solution

webContents.on("certificate-error", ((event, url, error, certificate, callback, isMainFrame)

### Alternatives Considered

Not sure

### Additional Information

Not sure
shell/browser/api/electron_api_app.cc
content::WebContents* web_contents,
    net::SSLCertRequestInfo* cert_request_info,
    net::ClientCertIdentityList identities,
    std::unique_ptr<content::ClientCertificateDelegate> delegate) {
  std::shared_ptr<content::ClientCertificateDelegate> shared_delegate(
      delegate.release());
  
  
  auto client_certs = net::CertificateList();
  for (const std::unique_ptr<net::ClientCertIdentity>& identity : identities)
    client_certs.emplace_back(identity->certificate());
  auto shared_identities =
      std::make_shared<net::ClientCertIdentityList>(std::move(identities));
  v8::Isolate* isolate = JavascriptEnvironment::GetIsolate();
  v8::HandleScope handle_scope(isolate);
  bool prevent_default =
      Emit("select-client-certificate",
           WebContents::FromOrCreate(isolate, web_contents),