BuiltIn: Enhance error reporting of comparison keywords especially when `ignore_case=True`

Case-insensitivity support added in RF 3.0.1 (#2439) works fine otherwise, but error reporting isn't always consistent. Sometimes error messages show the compared values turned to lower case, and in others they show the original values. It would probably be best to to always show the original values but add a note like (case-insensitive) to the message automatically.

There are also other inconsistencies in error reporting (e.g. do messages end with . or not) that can be fixed when refactoring error reporting in general. Having a better way to report errors involving large strings or containers with many items would be great too, but that is probably so big task that it would deserve a separate issue.