Use types to make sure value objects are correct

[Baptouuuu]

While closing down value objects it appeared LoadAverage wasn't safe. The possible error is now expressed as a type by Server::loadAverage() now returning an Attempt.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.93%. Comparing base (560b787) to head (d0f09ed).
Report is 24 commits behind head on develop.

Additional details and impacted files

@@            Coverage Diff             @@
##             develop      #13   +/-   ##
==========================================
  Coverage      99.93%   99.93%           
- Complexity       259      261    +2     
==========================================
  Files             55       55           
  Lines           1457     1516   +59     
==========================================
+ Hits            1456     1515   +59     
  Misses             1        1           

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