Changed error message shown when escript_embed_elixir is incorrect

First attempt at a fix for #2232. This is my first Elixir contribution so I may be completely wrong :). I have two questions regarding this pull request:

1) I'm assuming that just changing the error message string is good enough to fix #2232. Is that assumption correct?

2) I've written a test which seems to work fine. However, the test is throwing `warning: redefining module :escripttest`. I'm going to investigate this further but if anyone can advise on writing a better test, that would be great!