feat: return 404 for missing files (INFRA-735)

[siers]

Technically, the underlying function checks for file accessibility, but an internal server error would be inappropriate even then.

[linear]

INFRA-735 many errors from traefik

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 23.2%. Comparing base (4231f00) to head (a8ca8fc).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/SipiHttpServer.cpp	0.0%	0 Missing and 1 partial ⚠️

Additional details and impacted files

Files with missing lines	Coverage Δ
src/SipiHttpServer.cpp	22.2% <0.0%> (+0.7%)	⬆️

... and 5 files with indirect coverage changes

[SamuelBoerlin]

Thanks for the fix @siers ! :)