Surface metadata source failures instead of returning an empty result

[johnnyxmas]

Summary

BookInfoProxy.Search wraps the search proxy call in a blanket catch (Exception) that logs a warning and returns an empty list:

try
{
    result = _goodreadsSearchProxy.Search(query);
}
catch (Exception e)
{
    _logger.Warn(e, "Error searching for {0}", query);
    return new List<Book>();
}

So any failure to reach the metadata source renders as "no results found", which is indistinguishable from a genuine empty result. A metadata outage looks to the user like the author or book simply does not exist.

This is currently very visible, because the origin rate limits aggressively (see #8). Searching dune returns 25 results while Louis Armstrong, plain Armstrong and tolkien all return 0 — not because those have no books, but because the origin answered 429:

[Warn] BookInfoProxy: Error searching for louis armstrong
NzbDrone.Core.MetadataSource.Goodreads.GoodreadsException: Search for 'louis armstrong' failed.
Unable to communicate with metadata source.
 ---> NzbDrone.Common.Http.TooManyRequestsException: HTTP request failed:
      [429:TooManyRequests] [GET] at [https://hardcover.bookinfo.pro/search?q=louis%20armstrong]

Why rethrowing is the right fix

Two things indicate the exception was always meant to propagate:

1. GoodreadsException derives from NzbDroneClientException and is constructed with HttpStatusCode.ServiceUnavailable, so it is already designed to surface as a 503 with a message rather than be swallowed.

2. CandidateService already catches GoodreadsException to degrade gracefully during import identification:

catch (GoodreadsException e)
{
    _logger.Info(e, "Skipping author/title search due to Goodreads Error");
    remoteBooks = new List<Book>();
}

Because the blanket catch in BookInfoProxy runs first, that handler can never observe the exception — it is effectively dead code today.

Change

Rethrow GoodreadsException, keeping the existing empty-list fallback for other unexpected errors.

• Import identification is unaffected — CandidateService still catches it and falls back to an empty candidate list, which is what it already intended to do.
• Interactive search now reports the failure instead of silently showing nothing, surfacing as 503 "Unable to communicate with metadata source".

Considered and rejected: retrying with back-off

BookInfoProxy has a WaitUntilRetry helper used on other paths, but it calls Thread.Sleep on the Retry-After value. The origin currently returns retry-after: 132, so retrying a user-initiated search would block the request for over two minutes. Failing fast with an accurate message seemed clearly preferable for the interactive path; happy to revisit if you would rather have a bounded retry.

Test plan

• Verified GoodreadsException resolves in BookInfoProxy (using NzbDrone.Core.MetadataSource.Goodreads already present) and is the same type CandidateService catches
• Confirmed it derives from NzbDroneClientException with ServiceUnavailable, so it maps to a 503 rather than an unhandled 500
• Built on linux-x64 and deployed; rate-limited searches now report an error instead of an empty list