Better error messages / docs for registry references

[yuhan0]

I found out mostly through experimentation and examples that the :map and :ref schemas take only qualified keys and strings as registry references (as specified by malli.core/-reference?, it turns out)

The docs don't explicitly mention this requirement though, and the error messages produced by :map could be clearer:

(malli/schema [:map {:registry {:unqualified :any}}
               :unqualified])
;; => Execution error (UnsupportedOperationException) at malli.core/-parse-entries$-parse (core.cljc:185).
;;    count not supported on this type: Keyword

(malli/schema [:map {:registry {:unqualified :any}}
               [:unqualified]])
;; => Execution error (ExceptionInfo) at malli.impl.util/-fail! (util.cljc:16).
;;    :malli.core/invalid-schema {:schema nil}

Expected behavior:

Throw a :malli.core/invalid-ref error like the :ref schema does, including the invalid key in the error map.

(malli/schema [:ref :unqualified]
  {:registry (merge (malli/default-schemas) {:unqualified :any})})
;; => Execution error (ExceptionInfo) at malli.impl.util/-fail! (util.cljc:16).
;;    :malli.core/invalid-ref {:ref :unqualified}