Don't use `else` when `if` raises an exception

[lshandross]

Re loader.py:

python convention: we don't use else when if raises and exception. instead we let control flow to the next line. this makes for more readable code. this case is a bit atypical b/c the else is simply a return

    if as_of < datetime.date.fromisoformat("2025-09-17"):
      raise NotImplementedError("Functionality for loading NSSP data with specified as_of date is not implemented.")
    else:
      return self.load_nssp_from_cdc(
        disease=disease,
        as_of=as_of,
        drop_pandemic_seasons=drop_pandemic_seasons
      )

Originally posted by @matthewcornell in #25 (review)