fix: clear error when model file is missing or malformed (closes #20) - #83

Fixes #20.

Loading a missing or malformed model file previously ran nlohmann::json::parse on a failed/unopened stream, producing a confusing crash rather than a useful message:

Loading MACEKokkos model from '.../model.json' ... malloc_consolidate(): invalid chunk size

This adds two checks in load_from_json (in both mace.cpp and mace_kokkos.cpp):

• if the file cannot be opened, throw Could not open model file: <path>
• if parsing fails, throw Failed to parse model file '<path>': <detail>

Verified on macOS: loading a nonexistent path now throws Could not open model file: /tmp/does_not_exist.json instead of crashing.