API: status code 403 returned if new project would exceed limit

When the project limit is reached the user is not allowed to create new ones.
Instead of error code 404 the status code 403 (Forbidden) is returned with error
message via API.

lib/api/projects.rb

@@ -58,6 +58,9 @@ module Gitlab
         if @project.saved?
           present @project, with: Entities::Project
         else
+          if @project.errors[:limit_reached].present?
+            error!(@project.errors[:limit_reached], 403)
+          end
           not_found!
         end
       end