Processing .app.src.script expects a single value to be returned.

file:script returns a single value, not a list like file:consult.

Previously, one would get an unexpected_terms error.
```rebar_config:consult_file``` is running either ```consult_and_eval``` which returns {ok, Term} or ```file:consult``` which returns {ok, [Term]}, so we can't rely on the extension alone to know how the result will look like.

In my previous attempt I tried to "fix" ```rebar_config:consult_file```, but that affects compatibility when used for rebar.config files.

Also, for ".app", ```file:consult``` returns {ok, [Term]} so that needs to be destructured too.