#1061: add 404 message when loading non existing task

[StephanStrehlerCGI]

Description

Short description or comments

Reference

Issues: closes #1061

Check-List

• All Acceptance criteria of user story are met
• JUnit tests are written (60% CodeCov)
• Internal Review is maintained
• Documentations external and internal are completed
• Smoketest successful (Manual E2E-Test depending on Change)
• No Branch waste left
• Board is up-to-date
• Openshift environments are prepared (Secrets, etc.) and release-issue is maintained

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (5b256a3) 37.70% compared to head (7df05a2) 37.70%.

Additional details and impacted files

@@            Coverage Diff            @@
##                dev    #1071   +/-   ##
=========================================
  Coverage     37.70%   37.70%           
  Complexity     1261     1261           
=========================================
  Files           600      600           
  Lines          8696     8696           
  Branches        422      422           
=========================================
  Hits           3279     3279           
  Misses         5283     5283           
  Partials        134      134           

Flag	Coverage Δ
unittests	37.70% <ø> (ø)

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