Refactoring REST responses

[Thomsch]

Description

This pull request brings a few refactoring regarding where and how API responses are made to ensure a single point of maintenance and higher fidelity to the specification.

Changes

• Reusable error titles
• API responses are now all in responses.py
• Resources tests files have the same name suffix

Checklist

Tests

• All new and existing tests are passing ($ tox -e py38; replace 38 by your Python version if necessary)

Quality

• I have read the CONTRIBUTING doc
• My commits messages follow this format
• My code follows the style guidelines ($ tox -e lint)

[codecov-commenter]

Codecov Report

Merging #446 into develop will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop     #446   +/-   ##
========================================
  Coverage    49.47%   49.47%           
========================================
  Files           80       80           
  Lines        13808    13808           
  Branches       348      348           
========================================
  Hits          6832     6832           
  Misses        6960     6960           
  Partials        16       16           

Impacted Files	Coverage Δ
...ts/functional/serving/test_experiments_resource.py	100.00% <ø> (ø)
tests/functional/serving/test_trials_resource.py	100.00% <ø> (ø)

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