Merge branch 'ce-4135-remove-ee-specific-code-from-post-api-v4-projects-id-members' into 'master'

Backport part of EE's "Remove EE-specific code from `POST /api/v4/projects/:id/members`"

See merge request gitlab-org/gitlab-ce!16051

lib/api/members.rb

@@ -59,7 +59,9 @@ module API
 
           member = source.add_user(params[:user_id], params[:access_level], current_user: current_user, expires_at: params[:expires_at])
 
-          if member.persisted? && member.valid?
+          if !member
+            not_allowed! # This currently can only be reached in EE
+          elsif member.persisted? && member.valid?
             present member.user, with: Entities::Member, member: member
           else
             render_validation_error!(member)