Include the plugin channel name in the exception message

By: Phoenix616 <mail@moep.tv>

paper-api/src/main/java/org/bukkit/plugin/messaging/StandardMessenger.java

@@ -471,11 +471,11 @@ public class StandardMessenger implements Messenger {
             throw new ChannelNameTooLongException(channel);
         }
         if (channel.indexOf(':') == -1) {
-            throw new IllegalArgumentException("Channel must contain : separator");
+            throw new IllegalArgumentException("Channel must contain : separator (attempted to use " + channel + ")");
         }
         if (!channel.toLowerCase(Locale.ROOT).equals(channel)) {
             // TODO: use NamespacedKey validation here
-            throw new IllegalArgumentException("Channel must be entirely lowercase");
+            throw new IllegalArgumentException("Channel must be entirely lowercase (attempted to use " + channel + ")");
         }
         return channel;
     }