Unfortunately it doesn't look like there is a way to customize this message right now (without building Lemmy altered from source that is).
Relevant code: https://github.com/LemmyNet/lemmy-ui/blob/main/src/shared/components/app/error-page.tsx
Relevant translation file: https://github.com/LemmyNet/lemmy-translations/blob/main/translations/en.json (line 235 and 236 at time of writing).