[ToDo] Throw errors for invalid root nodes

[matthew-dean]

Right now, if you do something like:

@include borders();

If the return value is another call (instead of a result of a call), then it dumps the call as-is. We should make sure that that can't happen. Could just as simple a matter of checking the value of @include after evaluation. 🤔

[matthew-dean]

Doesn't check final "root" nodes, but added error handling to the @include eval