bug: running a query with $slot bind tokens through execute() reports an internal error (INT-025) #407

Summary

A query that uses $slot bind tokens is meant for prepare(). When it goes through execute() (or the REPL), the user gets an internal error that sounds like an engine bug:

(query [:find ?status
        :as-of $tx
        :valid-at $date
        :where [$entity :employment/status ?status]])
;; => Error: [INT-025] internal: unsubstituted :valid-at bind slot reached the executor

This example is copied from the wiki Datalog Reference (prepared queries section).

Expected

A clear user error with an API or parser code, for example: "query contains bind slots ($tx, $date, $entity); use db.prepare(...) and execute it with bind values". It should be raised before execution, in execute() or the parser. INT-0xx codes are for "should never happen" cases.

Notes

This does not change behavior; only the error code and message change. Add the new code to docs/ERROR_REFERENCE.md.

Found while checking the wiki examples; see #383.