Improve error detail when saving an incorrect LDAP config

[come-nc]

• Resolves: #

Summary

• Show what is the error when validating a configuration in the LDAP settings
• Check that user base and group base are under the root base

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)

[come-nc]

Configuration validation is weird.

1. For an invalid configuration, it happens on each get/set, so it will spam the logs
2. The configuration is saved before being checked, so it is saved even if not valid (there is not save button)
3. If you click "test configuration" too fast after a change, it actually tests the previous version of the configuration, before the change.

But all of that was already like that before this PR, so I still think the PR makes sense.