Added 502 error page for system startup

[MaxJa4]

Used https://github.com/HttpErrorPages to show a simple error 502 page, giving more information about the startup status of Tandoor.

This is typically shown in the time of web-container startup and during the migration phase.
Errors in the application (bugs) are usually 500 errors. These could be caught as well - if you want that (instead of the white flashbang error 500 page) I can add that too :)

If you'd like a different solution for this, feel free to let me know.

[vabene1111]

this is awesome, thank you very much, this will likely prevent lots of confusion. Maybe at some point we can add proper pages for all types of issues.