Failing zstd tests

[effigies]

The latest pyzstd release has changed the error type of:

>>> import io
>>> import pyzstd
>>> f = pyzstd.ZstdFile(io.BytesIO(b'bad'), mode='rb')
>>> f.read()

And tests are now failing.

@vanandrew Do you have time to look into this? I suspect what we want to do is coerce exceptions in Opener.read() to match expected exceptions from IOBase.read(), and possibly other methods.

[vanandrew]

@effigies I'll add it to my to-do list, but I might not get around to it until late next month.

[effigies]

No worries. I'll have a look at it in the next week or so if nobody gets there before me.