Report details of any errors during podcast downloads back to the user.

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Currently, only DNS resolution, HTTP 403 and HTTP 404 errors are caught by the Podcast Provider during the download process and reported to the user, any other download issue is reported as an 'Unknown Error'.

It would be better if any WebException encountered during the download process was passed back to the user so that they know why the failure occurred.

The exceptions reported by users all appear to have reasonably clear text as to why the failure occurred (at least for the innermost exception), so it would seem cleanest to report this to the user as the error details.