OPENNLP-1612 - DownloadUtil should validate checksum on download

[rzo1]

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Note:

Might help to identify issues with broken file downloads during EvalTests in #69

[mawiesne]

Idea:
Could instead throw a more specific exception (type) here to indicate a validation step has failed. Might be helpful for the caller to decide if a retry of the download shall be attempted or not.