public Path tmpPath() {
      return output.resolveSibling(output.getFileName() + "_inprogress");
    }
Starts downloading all resources in parallel, logging progress until complete.

@throws IllegalStateException if an error occurs downloading any resource, will be thrown after all resources
                              finish
planetiler-core/src/main/java/com/onthegomap/planetiler/util/Downloader.java