[constraint] more robust unicity constraint failures reporting for end-users Postgres or Sqlserver have limits on the index names (around resp. 64 and 128 characters). Because `logilab.database` encodes the `unique together` constraint rtypes in the index names, we sometimes get truncated index names, from which it is impossible to retrieve all rtypes. In the long run, the way such index are named should be changed. In the short term, we try to reduce the end-user confusion resulting from this design flaw: * in source/native, the regex filtering ``IntegrityError`` message does not impose an `_idx` suffix, which indeed may be absent (the result being an UI message that resembles a catastrophic failure), * also we avoid including a trailing " (double quote) from the error message * in entities/adapters, the well-named ``IUserFriendly`` adapter is made a bit smarter about how to handle missing rtypes. * the adapter also always produces a global message explaining the issue (and the fact that sometimes, the user is not shown all the relevant info) * i18n is updated Closes #2793789

CSS Coding Standards
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(Draft, to be continued)

:Naming: camelCase

Indentation rules
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- 2 espaces avant les propriétés

- pas d'espace avant les ":", un espace après

- 1 seul espace entre les différentes valeurs pour une même propriété


Documentation
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Please keep rules semantically linked grouped together, with a comment about
what they are for.

Recommendation
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- Try to use existing classes rather than introduce new ones

- Keep things as simple as possible while in the framework

- Think about later customization by application

- Avoid introducing a new CSS file for a few lines of CSS, at least while the
  framework doesn't include packing functionalities