fix: suppress misleading KeyError chain in _locate_site_class`

[blankirigaya]

In _locate_site_class (snp_data.py), when an unsupported site_class value is passed, the NotImplementedError was chained to exc — a KeyError from a cache dict lookup that is purely a normal cache-miss signal, not an error.

This caused misleading tracebacks like:

KeyError: ('some_region', None, 'BAD_CLASS')
During handling of the above exception, another exception occurred:
NotImplementedError: BAD_CLASS