Re-raise startup errors or shown them on 404 page?

[krassowski]

I was helping a user who had a startup error due to conda/pip conflict (#146 (comment)) and it struck me that they did not really post their tracback in the question, but instead titled it "404 : Not Found You are requesting a page that does not exist!". It kind of makes sense - they did see the 404 page and it was screaming louder than the traceback (which itself was already scrolled up as new messages appeared in the meantime - especially common when multiple jupyter_server extensions are installed!).

Would it be helpful if server re-raised critical startup errors from jupyterlab_server? Getting 404 page might confuse some users, as 404 page becomes the "large screaming message" rather than the traceback that is more useful. Or, possibly even better could the traceback be shown on the 404 page?

From the UX standpoint it seems important to have something more than 404 because users facing different issues all arrive to the same SO question and are annoyed that none of the proposed solutions (many of which actually target different issues that existed in the past) work for them (or maybe one, new solution does).

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