add error handling for load method

[halannhile]

Add error handling to the load method in base.py, specifically handling cases where the model file format might be incorrect or corrupted. Modifications include:

1. Checking the model file version and raising a ValueError if it's unsupported.
2. Verifying the format of lines when reading special tokens and merges, and raising ValueError for invalid lines.
3. Using a more specific RuntimeError for unexpected errors during file processing.