Add parsing of non-standard Error fields

[BoD]

I noticed GitHub's API returns Errors that have a non standard type field.

Looking at the spec, these are discouraged but still allowed - so I thought it could be useful to surface them.

In v2 these would come up in the extensions field - but I opted to add a specific field instead, to avoid potential clashes in the (improbable) event where fields with the same name appear at the top level and in extensions.

[BoD]

Note: also opened a ticket for GitHub about this.