Add new mode 'error' for DxcButton component

[Mil4n0r]

Checklist (Check off all the items before submitting)

• [x] Build process is done without errors. All tests pass in the /lib directory.
• [x] Self-reviewed the code before submitting.
• [ ] Meets accessibility standards.
• [x] Added/updated documentation to /website as needed.
• [ ] Added/updated tests as needed.

Purpose A clear description of the purpose of the change. Applicable in case this change is not linked to an issue.

Description A clear and concise description of the change.

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Additional context Add any other context about the change here.

Closes #1746

[Mil4n0r]

This functionality has to be discussed with the design team.