Refactor 2 functions to 1.
Catch attempts to (require r/g/b) at REPL prompt.

This uses current-module-name-resolver. It catches not only `(require
racket/gui/base)` at the REPL prompt, but also any other require that
transitively requires r/g/b.

Note that I can't figure out how to require r/g/b correctly here, inside
the running sandbox evaluator -- wrong namespace, not the main thread.
However I at least print an error and prevent it from happening
incorrectly.