fix(richtext-slate): Slate RichText contents error color in groups

[barthy-koeln]

Description

• [x] I have read and understand the CONTRIBUTING.md document in this repository.

When a group field has an error, its color is set to red. This affects the contents of Slate rich text fields. IMO, this will confuse users, who might think there's an error with this specific field:

This PR enforces a text color on the entire editor, similar to regular text fields.

Type of change

• [x] Bug fix (non-breaking change which fixes an issue)

Checklist:

• [ ] I have added tests that prove my fix is effective or that my feature works
• [x] Existing test suite passes locally with my changes
• [ ] I have made corresponding changes to the documentation

[denolfe]

@JarrodMFlesch Can you review?