const rollbarOnerror = window.onerror
window.onerror = function(message, source, lineno, colno, error) {
  rollbarOnerror(??????)
  if (!window.handledByRollbar) {
    // We do our own thing
  }
}


// Rollbar config
{
  ...
  onSendCallback: function(isUncaught) {
                  if (isUncaught) {
                    window.handledByRollbar = true
                    // Do happy path
                  }
                },
  ...
}