Source should return Source not found

Same as in https://github.com/Lambda-X/cljs-repl-web/issues/180, but with a different twist, as `lein browser-repl` is actually outputting `nil` for `(source not-required-symbol)`, or a weird empty `Error`.

The original message is [this](https://github.com/clojure/clojurescript/blob/master/src/main/clojure/cljs/repl.cljc#L1260). There is still something not working there.
