get-function() doesn't see functions defined in inner scopes

[nex3]

LibSass's get-function() function fails if it's passed a function name that's defined only in a nested scope. For example:

.scope {
  @function add-two($v) {@return $v + 2}
  $_: get-function(add-two);
}

This throws:

Error: Function not found: add-two
        on line 3:7 of test.scss, in function `get-function`
        from line 3:7 of test.scss
>>   $_: get-function(add-two);

   ------^

[nex3]

I'm adding a spec for this in sass/sass-spec#1345.

[xzyfer]

I have a fix for the error reported (#2846) but not the underlying issue of executing scoped function references.

Until the behavioural issue is resolved erring is better than producing broken output.