Add error on existing supplier name for current employee

spec/acceptance/suppliers/signup.feature

@@ -29,6 +29,15 @@ Feature: A supplier can sign up
     And I wait 1 second
     Then a new supplier with the existing employee should be created
     And the supplier should be redirected to the supplier '/pages/introduction' path
+
+  @javascript
+  Scenario: Existing signed in employee creates existing supplier name
+    Given there is a confirmed and open supplier
+    And I am signed in as supplier
+    When I visit the supplier signup path
+    And fill in the supplier signup form with an existing supplier name
+    And click on the supplier signup submit button
+    Then the new supplier signup for should have an error on 'supplier_name'
 #Scenarios
 #- non existing non signed in employee
 #- existing non signed in employee