Save the plugin form open in the CMS modal and wait for the modal to close. When the form is not valid the modal stays open, and the step fails with the errors the form reports.
Examples
When I save the plugin
When we save the plugin
And I save the plugin
When save the plugin
And we save the plugin
Pattern
When(/^(I |we )*save the plugin$/, …)
Part of @webship-js/django-cms-steps (github.com/webship/django-cms-steps), a step pack for Webship-js 2.0.x.