input_plugin: method init() returns errors with GError

Not used by any plugin currently, but this eliminates the g_error()
call in input_plugin_config(), so it's worth it.

src/input_plugin.h

@@ -35,10 +35,12 @@ struct input_plugin {
 	/**
 	 * Global initialization.  This method is called when MPD starts.
 	 *
+	 * @param error_r location to store the error occuring, or
+	 * NULL to ignore errors
 	 * @return true on success, false if the plugin should be
 	 * disabled
 	 */
-	bool (*init)(const struct config_param *param);
+	bool (*init)(const struct config_param *param, GError **error_r);
 
 	/**
 	 * Global deinitialization.  Called once before MPD shuts