Don't parse error pages as radio playlists (#5739)

# What does this implement/fix?

When we fetch a radio station's playlist file, we never looked at the
HTTP status code. So if the station's server was down or the playlist
had moved, we handed the returned error page straight to the playlist
parser — and every line of that HTML became a "track". Depending on the
provider that ended up as a bogus stream URL, or as a confusing "Empty
playlist" message in the log that pointed at the wrong problem.

Now a failing playlist request is reported as a failed request.

**Related issue (if applicable):**

- n/a

## Types of changes

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## Checklist

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where applicable.
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