Add onError callback for customizing error handling in handleAuth

[nicknisi]

Add an optional onError callback that can be used to customize how errors are handled in withAuth.

import { handleAuth } from '@workos-inc/authkit-nextjs';

export const GET = handleAuth({
  onError: ({ error, request }) => {
    // do something with the error
    return NextResponse.redirect('/somewhere');
  }
});

Fixes #192

[chamini2]

I know I don't work in WorkOS, but looks pretty straightforward to me! Thanks for the quick response.

Would appreciate if we can get a new version when this is merged!