Fixed nullcheck error message on null plugin (see previous commit)

By: Nathan Adams <dinnerbone@dinnerbone.com>

paper-api/src/main/java/org/bukkit/plugin/SimplePluginManager.java

@@ -383,7 +383,7 @@ public final class SimplePluginManager implements PluginManager {
             throw new IllegalArgumentException("Priority cannot be null");
         }
         if (plugin == null) {
-            throw new IllegalArgumentException("Priority cannot be null");
+            throw new IllegalArgumentException("Plugin cannot be null");
         }
         if (!plugin.isEnabled()) {
             throw new IllegalPluginAccessException("Plugin attempted to register " + type + " while not enabled");
@@ -412,7 +412,7 @@ public final class SimplePluginManager implements PluginManager {
             throw new IllegalArgumentException("Priority cannot be null");
         }
         if (plugin == null) {
-            throw new IllegalArgumentException("Priority cannot be null");
+            throw new IllegalArgumentException("Plugin cannot be null");
         }
         if (!plugin.isEnabled()) {
             throw new IllegalPluginAccessException("Plugin attempted to register " + type + " while not enabled");