Improve error response struct

[prodis]

Current error struct:

%Correios.CEP.Error{
  reason: "CEP NAO ENCONTRADO"
}

To be implemented:

%Correios.CEP.Error{
  type: :postal_code_not_found,
  message: "Postal code not found",
  reason: "CEP NAO ENCONTRADO"
}

Other know errors:

%Correios.CEP.Error{
  type: :postal_code_required,
  message: "Postal code is required",
  reason: nil
}

%Correios.CEP.Error{
  type: :postal_code_invalid,
  message: "Postal code is invalid",
  reason: nil
}

Unexpected errors returned by Correios API:

%Correios.CEP.Error{
  type: :unexpected_error,
  message: "Unexpected error",
  reason: "[Any error response returned by Correios API]"
}

[lcezermf]

Hey @prodis can I try to solve this one?

[prodis]

Yes, thanks. 🙂

I have added more details to the specification. Let me know if any doubts.

[lcezermf]

Just to let you know, I already start developing this fix, sorry for the lack of return.