Handle TrackInfo/Error in USPS response

Certain error responses were not being handled properly:

```
<?xml version="1.0"?>
<TrackResponse>
  <TrackInfo ID="9201990108549900172059">
    <Error>
      <Number>-2147219303</Number>
      <Description>Duplicate</Description>
      <HelpFile/>
      <HelpContext/>
    </Error>
    </TrackInfo>
</TrackResponse>
```

This PR adds the test that reproduces the problem, and adds the code that fixes it.

[fixes #48762537](https://www.pivotaltracker.com/story/show/48762537)

Fixes https://www.honeybadger.io/inspector/p/183/242098