### 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
std::string file_data, cert_path;
  std::u16string password;
  net::ScopedCERTCertificateList imported_certs;
  int rv = -1;
  std::string* cert_path_ptr = options.FindStringKey("certificate");
  if (cert_path_ptr)
    cert_path = *cert_path_ptr;
  std::string* pwd = options.FindStringKey("password");
  if (pwd)
    password = base::UTF8ToUTF16(*pwd);
  if (!cert_path.empty()) {
    if (base::ReadFileToString(base::FilePath(cert_path), &file_data)) {
      auto module = model->cert_db()->GetPrivateSlot();
      rv = model->ImportFromPKCS12(module.get(), file_data, password, true,
                                   &imported_certs);
      if (imported_certs.size() > 1) {
        auto it = imported_certs.begin();
        ++it;  
        for (; it != imported_certs.end(); ++it)
          rv &= model->SetCertTrust(it->get(), net::CA_CERT,
                                    net::NSSCertDatabase::TRUSTED_SSL);
      }
    }
  }
  return rv;
}
#endif
void OnIconDataAvailable(gin_helper::Promise<gfx::Image> promise,