Add github.com/cockroachdb/errors to hooks as alternative errors package

[ian-oneleet]

https://github.com/cockroachdb/errors is a popular alternative to the stdlib errors package. It provides alternative versions of errors.New, fmt.Errorf (which it calls errors.Newf), and errors.Join, among other functions.

Nilaway currently flags more false positives for codebases that use cockroachdb/errors instead of stdlib errors, because the built-in special cases for stdlib errors don't carry over to cockroachdb/errors. This PR duplicates the special cases in hook/assume_return.go and hook/replace_conditional.go so that they work with cockroachdb/errors.

Code was produced with LLM assistance (Claude).

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