Add better error for invalid playlists

src/invidious.cr

@@ -390,9 +390,8 @@ get "/playlist" do |env|
   page = env.params.query["page"]?.try &.to_i?
   page ||= 1
 
-  playlist = fetch_playlist(plid)
-
   begin
+    playlist = fetch_playlist(plid)
     videos = fetch_playlist_videos(plid, page, playlist.video_count)
   rescue ex
     error_message = ex.message
@@ -2823,9 +2822,9 @@ get "/api/v1/playlists/:plid" do |env|
   page = env.params.query["page"]?.try &.to_i?
   page ||= 1
 
-  playlist = fetch_playlist(plid)
   
   begin
+    playlist = fetch_playlist(plid)
     videos = fetch_playlist_videos(plid, page, playlist.video_count)
   rescue ex
     error_message = {"error" => "Playlist is empty"}.to_json

src/invidious/playlists.cr

@@ -142,6 +142,10 @@ def fetch_playlist(plid)
   end
 
   response = client.get("/playlist?list=#{plid}&disable_polymer=1")
+  if response.status_code != 200
+    raise "Invalid playlist."
+  end
+
   body = response.body.gsub(<<-END_BUTTON
   <button class="yt-uix-button yt-uix-button-size-default yt-uix-button-link yt-uix-expander-head playlist-description-expander yt-uix-inlineedit-ignore-edit" type="button" onclick=";return false;"><span class="yt-uix-button-content">  less <img alt="" src="/yts/img/pixel-vfl3z5WfW.gif">
   </span></button>