Catch exception for old boolean type

Since v1.2 (2016) the file representation for boolean types has changed. Files created before this version can't be read using h5py; the old representation is not compatible with h5py at all. This hasn't been an issue in general, but it came up recently (https://github.com/G-Node/nix-odML-converter/issues/26) during some old file conversions. The way to fix these would be to use NIX (C++) or HDF5 directly to change the format of older files. Since we can't automate that with NIXPy, we should catch the exception and provide a more useful error message.

Start by locating the code path that opens old NIX files and handles boolean-type read failures. Inspect the existing exception behavior, then verify that an incompatible pre-v1.2 file produces a clear, actionable error message rather than the raw exception.

Written by the indexing model from the issue text.