Raise different exception type in `decoding.py`?

[StokesMIDE]

IOError is raised several places in decoding.py; in hindsight, this is probably the wrong exception type, since it is an error in the data rather than an error accessing it. Some other exception type should be raised. While a ValueError would be more appropriate, maybe it should be a new subclass of IOError to maintain compatibility with existing code that uses ebmlite.

There is also one IOError raised in a similar situation in core.py that should probably be some other type.