Don't let ContentsHandler's internal 404 bubble up.

[Carreau]

See jupyterlab/jupyterlab#15153

ContentsHandler's get requests (and future HEAD requests #1327) should likely catch internal 404s and return a clean 404 with 'set_status()' and finish() this will avoid tornado to print tracebacks on non-existing files, which are often requested on purpose, like in SaveAs.

[Zsailer]

Yeah, I agree here, @Carreau. Thank you for raising here. I've labeled the issue with "good first issue" since this should be fairly strait forward.

[bloomsa]

can this be assigned to me? I'll give it a shot

[bloomsa]

FYI I think this can be closed now that #1357 is merged. let me know if I'm wrong

[Carreau]

Likely, let's close.