Error for TSV files that are not valid UTF-8

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According to Tabular files:

TSV files MUST be in UTF-8 encoding.

We currently don't validate this, which leads to situations when data is encoded in, e.g., ISO-8859 (https://github.com/OpenNeuroOrg/openneuro/issues/2515).

This one's a bit of a double-edged sword as it requires reading the entirety of every TSV file, which we've largely avoided to now.