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

## 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.
