Initialization using dclab.new_dataset may yield misleading error messages

[paulmueller]

When a user passes an argument e.g. access_key_id to new_dataset and boto3 is not installed, dclab should print a more helpful error message than "unexpected keword argument" when it tries to open the file as http.

[paulmueller]

It could also be the other way around (requests not available and wrong keyword arguments passed to another RTDCBase subclass).