feature: Add helper functions for working with errors

[nesv]

📝 Description

This pull request adds two functions to the public API for this package:

• ErrHasStatus(error, ...int) attempts to unwrap the provided error to an *Error and see if its status code matches any of the status codes provided.
• IsNotFound is a thin wrapper over ErrHasStatus(err, http.StatusNotFound) for a subjectively-common use case.

Implements #460

✔️ How to Test

make testunit