Rework exit notification for Instances

Pull the exit handler into its own method that's invoked if the startup
fails before the Factorio server is created.  Also move the metrics
clearing to the exit notification method to ensure it's also invoked.

slave.js

@@ -64,15 +64,25 @@ class Instance extends link.Link{
 		this.server.on("error", err => {
 			console.log(`Error in instance ${this.name}:`, err);
 		});
+	}
 
-		this.server.on("exit", err => {
-			link.messages.instanceStopped.send(this);
+	notifyExit() {
+		link.messages.instanceStopped.send(this);
 
-			// Notify plugins of exit
-			for (let pluginInstance of this.plugins.values()) {
-				pluginInstance.onExit();
+		// Clear metrics this instance is exporting
+		for (let collector of prometheus.defaultRegistry.collectors) {
+			if (
+				collector instanceof prometheus.ValueCollector
+				&& collector.metric.labels.includes('instance_id')
+			) {
+				collector.removeAll({ instance_id: String(this.config.get("instance.id")) });
 			}
-		});
+		}
+
+		// Notify plugins of exit
+		for (let pluginInstance of this.plugins.values()) {
+			pluginInstance.onExit();
+		}
 	}
 
 	async init(pluginInfos, slave) {
@@ -223,6 +233,7 @@ class Instance extends link.Link{
 			console.log("Clusterio | RCON connection established");
 		});
 
+		this.server.on("exit", () => this.notifyExit());
 		await this.server.start(saveName);
 		await this.server.disableAchievements()
 		await this.updateInstanceData();
@@ -254,15 +265,6 @@ class Instance extends link.Link{
 			await this.server.stop();
 		}
 
-		// Clear metrics this instance is exporting
-		for (let collector of prometheus.defaultRegistry.collectors) {
-			if (
-				collector instanceof prometheus.ValueCollector
-				&& collector.metric.labels.includes('instance_id')
-			) {
-				collector.removeAll({ instance_id: String(this.config.get("instance.id")) });
-			}
-		}
 	}
 
 	async getMetricsRequestHandler() {
@@ -300,6 +302,7 @@ class Instance extends link.Link{
 		console.log("Creating save .....");
 		await symlinkMods(this, "sharedMods", console);
 
+		this.server.on("exit", () => this.notifyExit());
 		await this.server.create("world");
 		console.log("Clusterio | Successfully created save");
 	}
@@ -600,13 +603,7 @@ class Slave extends link.Link {
 	async startInstanceRequestHandler(message, request) {
 		let instanceId = message.data.instance_id;
 		let instanceConnection = await this._connectInstance(instanceId);
-		try {
-			return await request.send(instanceConnection, message.data);
-
-		} catch (err) {
-			await this.stopInstance(instanceId);
-			throw err;
-		}
+		return await request.send(instanceConnection, message.data);
 	}
 
 	async createSaveRequestHandler(message, request) {