Use @error rather than @warn

[KittyGiraudel]

Instead of @warning when something goes wrong regarding the Sass logic, I'd advise to throw an error with @error. There is no reason to let the script run when there is an error, so you better throw an actual error.

For instance:

@function ext-get-option($option) {
  @if not map-has-key($ext-opt, $option) {
    @error 'The option `#{$option}` is not defined or part of the options.';
  }

  @return map-get($ext-opt, $option);
}

[jaicab]

Fixed on #8.