node-sass compat: async errors for sass functions

[chriseppstein]

In node-sass you return a value of type sass.types.Error if you want to communicate a failure that occurred during function execution back to the sass compiler. In dart-sass, there's no sass.types.Error constructor. According to @nex3, dart-sass expects an instance of the standard js Error object, but returning a standard error object to node-sass fails with error in C function eyeglass-fs-info: A SassValue object was expected.

[chriseppstein]

example code:

let sassOptions = {
  "functions": {
    "raise-error($wait-time)": function(waitTime, cb) {
      setTimeout(function() {
        cb(new sass.types.Error("An Error!"));
      }, waitTime.getValue());
    }
  }
};

[nex3]

The hilarious thing here is that passing an Error to an importer's callback works in Node Sass, but passing a sass.types.Error does not.