Warn when using Guava Subscribe annotation.

Additionally, registration of listeners will not be fatal if an
exception was thrown.

proxy/src/main/java/com/velocitypowered/proxy/VelocityServer.java

@@ -225,11 +225,16 @@ public class VelocityServer implements ProxyServer {
       logger.error("Couldn't load plugins", e);
     }
 
-    // Register the plugin main classes so that we may proceed with firing the proxy initialize event
+    // Register the plugin main classes so that we can fire the proxy initialize event
     for (PluginContainer plugin : pluginManager.getPlugins()) {
       Optional<?> instance = plugin.getInstance();
       if (instance.isPresent()) {
-        eventManager.register(instance.get(), instance.get());
+        try {
+          eventManager.register(instance.get(), instance.get());
+        } catch (Exception e) {
+          logger.error("Unable to register plugin listener for {}",
+              plugin.getDescription().getName(), e);
+        }
       }
     }
 

proxy/src/main/java/com/velocitypowered/proxy/plugin/VelocityEventManager.java

@@ -70,6 +70,15 @@ public class VelocityEventManager implements EventManager {
     if (plugin == listener && registeredListenersByPlugin.containsEntry(plugin, plugin)) {
       throw new IllegalArgumentException("The plugin main instance is automatically registered.");
     }
+
+    for (Method method : listener.getClass().getDeclaredMethods()) {
+      if (method.isAnnotationPresent(com.google.common.eventbus.Subscribe.class)) {
+        throw new IllegalArgumentException("Method " + listener.getClass().getName() + "#"
+          + method.getName() + " has a Guava @Subscribe annotation. Use the Velocity @Subscribe "
+          + "annotation instead.");
+      }
+    }
+
     registeredListenersByPlugin.put(plugin, listener);
     methodAdapter.register(listener);
   }