Replace `variant="error"` with `variant="danger"`

[glenngijsberts]

Improvement description

For our Alert component we're using the variant error, while we use danger for other components such as our Button component. It would be nice to align the two, so I would go for replacing error with danger.

Expected behaviour

Deprecate "error" variant for the Alert component & add "danger" variant instead. Use the "danger" variant inside the stories.