bittorrent

Creating a new issue based on lbryio/lbry-sdk#877

• Update ensure_directory_exists() to check that the directory is writable by the current process.
• If it's not, return a useful error
• Add tests to cover the case when
  • download directory does not exist
  • download dir exists but is not writable
  • dir exists an

Because in CliClient.resume() we don't account for the case, when there is an exception in one of the async processing stages:

void resume() {
	...
	try {
		client.startAsync(state -> {
			printer.ifPresent(p -> p.print(state));
			if (!options.shouldSeedAfterDownloaded() && state.getPiecesRemaining() == 0) {
				runtime.shutdown();
			}
		}, 1000); // <-- CompletableFuture.w