Use a custom channel not found exception.

osu.Game/Graphics/Containers/LinkFlowContainer.cs

@@ -84,7 +84,7 @@ public void AddLink(string text, string url, LinkAction linkType = LinkAction.Ex
                             {
                                 channelManager.OpenChannel(linkArgument);
                             }
-                            catch (ArgumentException)
+                            catch (ChannelNotFoundException)
                             {
                                 //channel was not found
                             }

osu.Game/Online/Chat/ChannelManager.cs

@@ -60,7 +60,7 @@ public void OpenChannel(string name)
                 throw new ArgumentNullException(nameof(name));
 
             CurrentChannel.Value = AvailableChannels.FirstOrDefault(c => c.Name == name)
-                                   ?? throw new ArgumentException($"Channel {name} was not found.");
+                                   ?? throw new ChannelNotFoundException(name);
         }
 
         public void OpenUserChannel(User user)
@@ -298,4 +298,14 @@ private void load(IAPIProvider api)
             api.Register(this);
         }
     }
+
+
+    public class ChannelNotFoundException : Exception
+    {
+        public ChannelNotFoundException(string channelName)
+            : base($"A channel with the name {channelName} could not be found.")
+        {
+
+        }
+    }
 }