[otbn] Add functions to classify err_bits

Different parts of `err_bits` fall into a few different classes

* fatal errors
* software errors
* recoverable errors (currently same as software errors but might not always overlap)

Having some functions such as `is_fatal_err` in `otbn_pkg` can help keep this consistent across everything that needs to take an `err_bits` and classify the kind of error and allow easy alteration of the various classes.

Start in the `otbn_pkg` definitions and trace how `err_bits` is currently interpreted across the project. Identify the fatal, software, and recoverable classifications and the existing users that should share them. Done means consistent classification functions are available in `otbn_pkg` and relevant callers use them, with tests covering the defined classes.

Written by the indexing model from the issue text.