Add documentation about correct way of handling non-existent content in importer function

[ubenzer]

Hello,

I am trying to use node-saas with custom importer function. The reason I am doing this is, I don't directly read from file, instead I am compiling saas from in-memory strings.

In importer function,
in case I can't find the partial how should I handle this error so it fails gracefully?
https://github.com/sass/node-sass#importer-starting-from-v2

Should I return null; or done(null); ? Or should I simply throw a new Error("File not found") ?

[am11]

Would you like to send a PR based on your findings and the discussion that transpired here: #615?

[am11]

This is fixed by c481f30.

The new implementation (#817) lets you use the API as follow:

var sass = require('node-sass');

var options = {
  file: 'path/to/file.scss',
  importer: functions (url, prev, done) {

      try {
        var response = {
        file: processPaths(url, prev)
        };
        done(response)
      } catch (_) {
        done(new Error('my faviourite error message'));
      }
  }
};

sass.render(options, function (err, response) {
  if (err) {
    console.error(err); return;
  }
});