Validate provided status before updating trial

[bouthilx]

Description

With the user interface it is possible that users try to set trials with
invalid status. We should verify the validity of the status before
potentially corrupting the database.

Changes

Add validity checks in storage.set_trial_status to make sure status is valid.

Checklist

Tests

• I added corresponding tests for bug fixes and new features. If possible, the tests fail without the changes
• All new and existing tests are passing ($ tox -e py38; replace 38 by your Python version if necessary)

Documentation

• I have updated the relevant documentation related to my changes

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 #399 into develop will increase coverage by 0.01%.
The diff coverage is 57.14%.

@@             Coverage Diff             @@
##           develop     #399      +/-   ##
===========================================
+ Coverage    46.61%   46.62%   +0.01%     
===========================================
  Files           67       67              
  Lines        12754    12768      +14     
  Branches       317      317              
===========================================
+ Hits          5945     5953       +8     
- Misses        6784     6790       +6     
  Partials        25       25              

Impacted Files	Coverage Δ
tests/unittests/storage/test_storage.py	17.52% <14.28%> (-0.07%)	⬇️
tests/unittests/client/test_experiment_client.py	58.86% <100.00%> (+0.56%)	⬆️

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[Thomsch]

We should think about progressively renaming stati in statuses (https://www.merriam-webster.com/dictionary/status)