There's no alert if new file name is wrong on multiple files

[felipecalvo]

If new file doesn't contain . or name already exists, it just fails silently - there's no way to know something was wrong.

Since we're already using a prompt for the user to input the name, we might as well use an alert to explain what failed

See https://github.com/mumuki/mumuki-laboratory/blob/f41f508b90af446dd18a4d2884946322a77f726b/app/assets/javascripts/mumuki_laboratory/application/multiple-files.js#L141