Fix Shikimori ignoring missing remote entry

The user would see no indication that Shikimori could not properly
refresh the track from the remote. This change causes the error Toast
notification to pop up with the following message
"Shikimori error: Could not find manga".

This is analogous to Kitsu and Bangumi.

app/src/main/java/eu/kanade/tachiyomi/data/track/shikimori/Shikimori.kt

@@ -94,7 +94,7 @@ class Shikimori(id: Long) : BaseTracker(id, "Shikimori"), DeletableTracker {
             track.library_id = remoteTrack.library_id
             track.copyPersonalFrom(remoteTrack)
             track.total_chapters = remoteTrack.total_chapters
-        }
+        } ?: throw Exception("Could not find manga")
         return track
     }