return error when failing to load plugin list

[flaix]

When the plugin registry is unreachable or returns an empty list, just an error is shown in the logs. It would be better to return an error through the ssh command, so that the user/admin directly sees that something is amiss.

2022-12-09 20:46:40 [ERROR] Failed to retrieve plugins.json from http://plugins.gitblit.com/plugins.json
java.io.FileNotFoundException: http://gitblit.github.io/gitblit-registry/plugins.json
	at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
	at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:62)
	at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
	at java.lang.reflect.Constructor.newInstance(Constructor.java:423)
	at sun.net.www.protocol.http.HttpURLConnection$10.run(HttpURLConnection.java:1950)
	at sun.net.www.protocol.http.HttpURLConnection$10.run(HttpURLConnection.java:1945)
	at java.security.AccessController.doPrivileged(Native Method)
	at sun.net.www.protocol.http.HttpURLConnection.getChainedException(HttpURLConnection.java:1944)
	at sun.net.www.protocol.http.HttpURLConnection.getInputStream0(HttpURLConnection.java:1514)
	at sun.net.www.protocol.http.HttpURLConnection.getInputStream(HttpURLConnection.java:1498)
	at com.gitblit.manager.PluginManager.downloadFile(PluginManager.java:536)