What about adding a .Level(slog.Level) method to the error builder?

Pros:

• When I build an error in a nested function, the caller cannot differentiate between an acceptable error (warning?) and a blocking error (error).
• An HTTP status 404 is a warning while a 500 is an error, but both are considered errors by the HTTP frameworks.

Cons:

• The log level is a logger thing, while this library provides an error builder. The separation of concern is not fully respected.