message: add warning if mapdir is used with emscripten
diff: @@ -445,6 +445,9 @@ fn execute_wasm(options: &Run) -> Result<(), String> {
  .instantiate(&import_object)
  .map_err(|e| format!("Can't instantiate module: {:?}", e))?;
  + if !mapped_dirs.is_empty() {
  + eprintln!("WARN: mapdir is not implemented for emscripten targets");
  + }
  wasmer_emscripten::run_emscripten_instance(
  &module,
  &mut instance,
  