Handle partial directory upload failure

[lukehedger]

When uploading a directory to http://swarm-gateways.net it is possible that the upload will only partially complete.

Taking the readme example Uploads/With JSON/Directory:

const dir = {
  "/foo.txt": {type: "text/plain", data: "file 0"},
  "/bar.txt": {type: "text/plain", data: "file 1"}
};
swarm.upload(dir).then(hash => {
  console.log("Uploaded directory. Address:", hash);
});

The console log will read:

PUT http://swarm-gateways.net/bzz:/${hash}/bar.txt 500 (Internal Server Error)

Uploaded directory. Address: error creating manifest: error loading manifest ${hash}: Manifest not Found

The first file foo.txt can then be found at http://swarm-gateways.net/bzz:/${hash}/foo.txt but bar.txt will not exist.

The fact that this error occurs is another matter but it could possibly be handled better by swarm-js with a more informative response?