Fix faultcode+faultstring and get UPnPError details

[jybp]

👋 A small PR that does two things:

1. Fixes the parsing of faultcode/faultstring fields.
   The change was introduced in this PR but I'm not sure why? Improve error handling #38
   You've also properly set them in v2alpha: https://github.com/huin/goupnp/blob/main/v2alpha/soap/envelope/envelope.go#L23:L24
   And the specs clearly specify this case: https://openconnectivity.org/upnp-specs/UPnP-arch-DeviceArchitecture-v2.0-20200417.pdf
   See page 74 as well as examples pages 83 & 84.

2. Parses some details of the optional UPnPError element. This will allow consumers that want to inspect the error to be able to easily retrieve errorCode and errorDescription. The Raw field is still there as a catch-all.

[jybp]

cc @huin

[huin]

Thanks, looks good.

There's a slight risk that existing code is pulling UPnPError out of Raw. I wonder if we should instead provide parsing methods on the fault type to extract this as a named struct, so that existing code won't change behaviour as a result. Otherwise this change would be good for a major version.

[jybp]

What do you mean by risk of code pulling out from Raw?
Raw should always contain the whole content of detail, no matter if Errorcode/ErrorDescription have been set (like in the test). The change shouldn't be disruptive to existing codebases that don't know about the new UPnPError exported field, but rely on Raw.

If you think just adding fields to SOAPFaultError still requires a major version, we can provide a helper like you suggested: c460521
Otherwise, the last commit can be reverted.

[huin]

What do you mean by risk of code pulling out from Raw?

Yes.

But I hadn't appreciated that that's how the XML unpacking worked - thanks for explaining that. In that case we can go with your original changes.

[jybp]

Cool, I removed that last change then. LMK when that can be merged & tagged 👍

[huin]

That's merged in. Thanks :)

[jybp]

Any chance a v1.2.0 could be introduced with this change ? :)

[huin]

Heh, woops. I actually drafted the release, and then failed to publish it. Done. :)