:zen/tags keywords instead of symbols doesn't return error

Steps to reproduce:
```
 (do
   (def ztx (zen.core/new-context))
   (zen.core/load-ns ztx '{:ns myns foo {:zen/tags #{:zen/schema} :type :foo}})
   (zen.core/errors ztx))
;; => []
```

Urgency: low