Validate settings on startup with JSON schema.

lib/controller.js

@@ -70,6 +70,18 @@ class Controller {
     }
 
     async start() {
+        const settingsErrors = settings.validate();
+        if (settingsErrors) {
+            logger.error(`Refusing to start, configuration.yaml is not valid, found the following errors:`);
+            for (const error of settingsErrors) {
+                logger.error(`\t - ${error}`);
+            }
+            logger.error(
+                `If you don't know how to solve this, read https://www.zigbee2mqtt.io/configuration/configuration.html`
+            );
+            process.exit(1);
+        }
+
         logger.info(`Logging to directory: '${logger.directory}'`);
         logger.cleanup();
         this.state.start();
@@ -242,7 +254,7 @@ class Controller {
         }
 
         const options = {
-            retain: entity.settings.hasOwnProperty('retain') ? entity.settings.retain : false,
+            retain: entity.settings.retain,
             qos: entity.settings.hasOwnProperty('qos') ? entity.settings.qos : 0,
         };