fix(api): return 404 status codes on "not found" pages

[adrienjoly]

Replaces the following log entries found in Datadog:

• postViewer error: POST_NOT_FOUND (cf logs)
• rendering server error page 404 for GET * (cf logs)

Contributes to #638.

[codacy-production]

Coverage summary from Codacy

See diff coverage on Codacy

Coverage variation	Diff coverage
-0.11%	∅

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (12f5b82)	5066	2986	58.94%
Head commit (b5898d2)	5116 (+50)	3010 (+24)	58.84% (-0.11%)

Coverage variation is the difference between the coverage for the head and common ancestor commits of the pull request branch: <coverage of head commit> - <coverage of common ancestor commit>

Diff coverage details

	Coverable lines	Covered lines	Diff coverage
Pull request (#674)	0	0	∅ (not applicable)

Diff coverage is the percentage of lines that are covered by tests out of the coverable lines that the pull request added or modified: <covered lines added or modified>/<coverable lines added or modified> * 100%

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