Improve error messages when uploading data and write detailed HowTo

[dalonsoa]

Error message indicating that the upload failed. This might be for a number of reasons, and I agree that the message is not very informative. Even if the format object is correct, it might not have any variable associated with it. For that you need to create a Classification, linking one or more variables with a format. That you have this problem points to a need for more clear error messages and a more detailed HowTo in the documentation for uploading data.

Originally posted by @dalonsoa in #332 (comment)