Assert the login screen shows, or does not show, its error notice (#login_error), optionally containing a text such as "incorrect" or "Unknown username". The notice is what a wrong password, an unknown user or an empty form produces.
Examples
Then I should see a login error
And we should see a sign-in error message
Then should see a login error "incorrect"
Then I should not see a login error
But we should not see an error message "incorrect"
Pattern
Then(/^(I |we )*should( not)* see an? (?:login |sign[- ]?in )?error(?: message)?(?: "([^"]*)")?$/, …)
Part of @webship-js/wordpress-steps (github.com/webship/wordpress-steps), a step pack for Webship-js 2.0.x.