Better error message for non-existing dataset #1111

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FileNotFoundError                         Traceback (most recent call last)
/usr/local/lib/python3.7/dist-packages/openml/datasets/functions.py in _get_dataset_description(did_cache_dir, dataset_id)
    948     try:
--> 949         with io.open(description_file, encoding="utf8") as fh:
    950             dataset_xml = fh.read()

FileNotFoundError: [Errno 2] No such file or directory: '/root/.cache/openml/org/openml/www/datasets/987987987/description.xml'

During handling of the above exception, another exception occurred:

OpenMLServerException                     Traceback (most recent call last)
7 frames
/usr/local/lib/python3.7/dist-packages/openml/_api_calls.py in __check_response(response, url, file_elements)
    282 def __check_response(response, url, file_elements):
    283     if response.status_code != 200:
--> 284         raise __parse_server_exception(response, url, file_elements=file_elements)
    285     elif (
    286         "Content-Encoding" not in response.headers or response.headers["Content-Encoding"] != "gzip"

OpenMLServerException: https://www.openml.org/api/v1/xml/data/987987987 returned code 111: Unknown dataset - None