Fixed missing key

src/NadekoBot/Modules/Searches/Searches.cs

@@ -515,7 +515,10 @@ namespace NadekoBot.Modules.Searches
                 var sense = data.Results.FirstOrDefault(x => x.Senses?[0].Definition != null)?.Senses[0];
 
                 if (sense?.Definition == null)
+                {
+                    await ReplyErrorLocalized("define_unknown").ConfigureAwait(false);
                     return;
+                }
 
                 var definition = sense.Definition.ToString();
                 if (!(sense.Definition is string))

src/NadekoBot/_strings/ResponseStrings.en-US.json

@@ -446,6 +446,7 @@
   "music_skipped_to": "Skipped to `{0}:{1}`",
   "music_songs_shuffled": "Songs shuffled",
   "music_song_moved": "Song moved",
+  "music_song_not_found":  "No song found.",
   "music_time_format": "{0}h {1}m {2}s",
   "music_to_position": "To position",
   "music_unlimited": "unlimited",
@@ -522,6 +523,7 @@
   "searches_cost": "Cost",
   "searches_date": "Date",
   "searches_define": "Define:",
+  "searches_define_unknown":  "Can't find the definition for that term.",
   "searches_dropped": "Dropped",
   "searches_episodes": "Episodes",
   "searches_error_occured": "Error occurred.",