Enable casting custom errors in Go

[helderco]

Follow up to #5184

Instead of having to unwrap a custom error:

var e *dagger.ExecError
if errors.As(err, &e) {
    return e.ExitCode
}

This allows directly casting:

if e, ok := err.(*dagger.ExecError); ok {
    return e.ExitCode
}

[vito]

I think this is fine, but folks should be using errors.As anyway; directly asserting on an error's type is super brittle. Was this causing issues?

[helderco]

Was this causing issues?

No, it just seemed to me based on some shared bits of code that people might expect to do it that way. Not married to it in any way 🙂

[vito]

All good! Just curious. :) It makes sense to me to not directly wrap our own expected errors anyhow.