Change Slug Validator Error Message

[robinmanuelthiel]

https://github.com/wemogy/libs-core/blob/bd6e1797aee3a79951e0da851da136e403325812/src/Wemogy.Core/Validation/Extensions/RuleBuilderInitialExtensions.cs#L45C77-L45C77

As we use the Slug Validator also on some IDs, we should not use the term "Slug" in the error message

[robinmanuelthiel]

Also, when the Slug is empty, we return a confusing error message saying "No special Chars"....