[bugfix] As these are plugins log a Warning and not an Error if the plugin is unavailable

Author: adamretter

exist-core/src/main/java/org/exist/plugin/PluginsManagerImpl.java

@@ -241,7 +241,7 @@ public void addPlugin(final String className) {
                 // NOTE: must set interrupted flag
                 Thread.currentThread().interrupt();
             }
-            LOG.error(e);
+            LOG.warn(e);
 		}
 	}