Expose Error for user

**Is your feature request related to a problem? Please describe.**
Hello, I'm currently trying to use pulser-client-go, and it seems very useful and easy.

However, producer can occur error and it is really ok. But, in some cases, I want to do some specific behavior based on its error content.

But, this package doesn't expose Error's field and doesn't offer any method or easy way to compare errors. So, I need to rewrite errors and write some comparing logic for my business logic.

I think it is not good pattern. If it is ok, please expose errors.

**Describe the solution you'd like**

Expose errors, and let users be able to use error.

No file or test is named in the issue. Start by tracing producer error handling in the Go client and reviewing how errors are currently exposed; done means callers can inspect error content and compare errors without rewriting them.

Written by the indexing model from the issue text.