Merge "if a download dir doesn't exist, create it or fail fast"

         * @return this object

         */

        public Request setDestinationInExternalPublicDir(String dirType, String subPath) {

            setDestinationFromBase(Environment.getExternalStoragePublicDirectory(dirType), subPath);

            File file = Environment.getExternalStoragePublicDirectory(dirType);

            if (file.exists()) {

                if (!file.isDirectory()) {

                    throw new IllegalStateException(file.getAbsolutePath() +

                            " already exists and is not a directory");

                }

            } else {

                if (!file.mkdir()) {

                    throw new IllegalStateException("Unable to create directory: "+

                            file.getAbsolutePath());

                }

            }

            setDestinationFromBase(file, subPath);

            return this;

        }