Check more locations for dataset file and fail if comparison files mi…

[JonathanTripp]

Searching for "dataset.csv" and "metrics.csv" used ValueError in the except block, but Azure ML now throws UserErrorException
if the file does not exist, so add that to the list to check.

Also, if the files are missing and they are expected then raise a new Exception and do not silently fail in the MLRunner.

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[ant0nsc]

this whole block from line 202 onwards, not sure this adds much value. Do we not yet have all the information in the logs, from line 191 and 179? Also, in which cases is download__outputs_from_run going to return something that is not None, but the file does not actually exist? This would simplify the checks.