Two small fixes from fuzz testing.

[wesc]

1. When docx-parser::MarkdownDocument receives corrupt or empty input, its from_file panics. We now catch these panics and convert them into Errors in FileProcessor::process_file.
2. Statistical functions panic when given bad inputs, potentially causing _find_optimal_threshold to crash the process. _find_optimal_threshold now handles bad input safely, and we remove the panics from the math functions.