upload.dataset() returns 409 error if uploading a duplicate

[andrie]

This throws a 409 error:

upload.dataset(airquality, ws, name = "Air quality")
upload.dataset(airquality, ws, name = "Air quality")

Error in upload.dataset(airquality, ws, name = "Air quality") : 
  HTTP 409 Request ID: f90c779a-1b69-4835-8090-1262699a67e9 2015-11-24 18:11:23Z

Suggestions:

• Test if name already exists in datasets(ws), before upload and stop with helpful message
• If that fails, test for 409 error and decode with a helpful message

Reported by @sdgilley