Improve message for grab errors due to no matching tags

Co-authored-by: zakary <zak@ary.dev>
(cherry picked from commit df672487cf1d5f067849367a2bfb0068defc315d)

Closes #3814

src/NzbDrone.Core.Test/Download/DownloadClientProviderFixture.cs

@@ -201,16 +201,8 @@ namespace NzbDrone.Core.Test.Download
             var clientTags = new HashSet<int> { 1 };
 
             WithTorrentClient(0, clientTags);
-            WithTorrentClient(0, clientTags);
-            WithTorrentClient(0, clientTags);
-            WithTorrentClient(0, clientTags);
-
-            var client1 = Subject.GetDownloadClient(DownloadProtocol.Torrent, 0, false, seriesTags);
-            var client2 = Subject.GetDownloadClient(DownloadProtocol.Torrent, 0, false, seriesTags);
-            var client3 = Subject.GetDownloadClient(DownloadProtocol.Torrent, 0, false, seriesTags);
-            var client4 = Subject.GetDownloadClient(DownloadProtocol.Torrent, 0, false, seriesTags);
 
-            Subject.GetDownloadClient(DownloadProtocol.Torrent, 0, false, seriesTags).Should().BeNull();
+            Assert.Throws<DownloadClientUnavailableException>(() => Subject.GetDownloadClient(DownloadProtocol.Torrent, 0, false, seriesTags));
         }
 
         [Test]

src/NzbDrone.Core/Download/DownloadClientProvider.cs

@@ -41,18 +41,23 @@ namespace NzbDrone.Core.Download
             var blockedProviders = new HashSet<int>(_downloadClientStatusService.GetBlockedProviders().Select(v => v.ProviderId));
             var availableProviders = _downloadClientFactory.GetAvailableProviders().Where(v => v.Protocol == downloadProtocol).ToList();
 
-            if (tags != null)
+            if (!availableProviders.Any())
+            {
+                return null;
+            }
+
+            if (tags is { Count: > 0 })
             {
                 var matchingTagsClients = availableProviders.Where(i => i.Definition.Tags.Intersect(tags).Any()).ToList();
 
                 availableProviders = matchingTagsClients.Count > 0 ?
                     matchingTagsClients :
                     availableProviders.Where(i => i.Definition.Tags.Empty()).ToList();
-            }
 
-            if (!availableProviders.Any())
+                if (!availableProviders.Any())
-            {
+                {
-                return null;
+                    throw new DownloadClientUnavailableException("No download client was found without tags or a matching author tag. Please check your settings.");
+                }
             }
 
             if (indexerId > 0)