Rework missing/outlier data event description

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (e8558bc) 95.79% compared to head (34d7d89) 95.79%. Report is 3 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #211 +/- ## ======================================= Coverage 95.79% 95.79% ======================================= Files 42 42 Lines 2708 2708 Branches 339 339 ======================================= Hits 2594 2594 Misses 90 90 Partials 24 24 ``` | [Files Changed](https://app.codecov.io/gh/BEMServer/bemserver-core/pull/211?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=BEMServer) | Coverage Δ | | |---|---|---| | [bemserver\_core/scheduled\_tasks/check\_missing.py](https://app.codecov.io/gh/BEMServer/bemserver-core/pull/211?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=BEMServer#diff-YmVtc2VydmVyX2NvcmUvc2NoZWR1bGVkX3Rhc2tzL2NoZWNrX21pc3NpbmcucHk=) | `90.00% <ø> (ø)` | | | [bemserver\_core/\_\_init\_\_.py](https://app.codecov.io/gh/BEMServer/bemserver-core/pull/211?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=BEMServer#diff-YmVtc2VydmVyX2NvcmUvX19pbml0X18ucHk=) | `100.00% <100.00%> (ø)` | | | [bemserver\_core/scheduled\_tasks/check\_outliers.py](https://app.codecov.io/gh/BEMServer/bemserver-core/pull/211?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=BEMServer#diff-YmVtc2VydmVyX2NvcmUvc2NoZWR1bGVkX3Rhc2tzL2NoZWNrX291dGxpZXJzLnB5) | `90.38% <100.00%> (ø)` | |

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