Fix AttributeError exception

[fedorov]

I think this line should be catching AtrributeError, not KeyError exception: https://github.com/MGHComputationalPathology/highdicom/blob/master/src/highdicom/legacy/sop.py#L361

[hackermd]

Thanks @fedorov for catching this bug. In an earlier implementation, I accessed the underlying data element in the data set using the tag rather than the attribute keyword and forgot to change this afterwards. Fixed via d0357d0.

We need to add unit tests for legacy conversion..