Add option to throw on non-empty RCON responses

Add option to FactorioServer.sendRcon to throw on non-empty responses
from Factorio.  This is useful for detecting errors returned by commands
that should not have a response.

lib/factorio/server.js

@@ -625,15 +625,22 @@ class FactorioServer extends events.EventEmitter {
 	 * wait until it is establied and then send the message.
 	 *
 	 * @param {string} message - message to send to server over RCON.
+	 * @param {boolean} expectEmpty -
+	 *     if true throw if the response is not empty.  Useful for detecting
+	 *     errors that might have been sent in response.
 	 * @returns {string} response from server.
 	 */
-	async sendRcon(message) {
+	async sendRcon(message, expectEmpty) {
 		this._check(["running"])
 		if (!this._rconReady) {
 			await events.once(this, 'rcon-ready');
 		}
 
-		return await this._rconClient.send(message);
+		let response = await this._rconClient.send(message);
+		if (expectEmpty && response !== '') {
+			throw new Error(`Expected empty response but got "${response}"`);
+		}
+		return response;
 	}
 
 	/**

plugins/global_chat/instance.js

@@ -34,7 +34,7 @@ class InstancePlugin extends plugin.BaseInstancePlugin {
 	async chatEventHandler(message) {
 		// TODO check if cross server chat is enabled
 		let content = `[${message.data.instance_name}] ${removeTags(message.data.content)}`;
-		await this.instance.server.sendRcon(`/sc game.print('${escapeLuaString(content)}')`);
+		await this.instance.server.sendRcon(`/sc game.print('${escapeLuaString(content)}')`, true);
 	}
 
 	async onOutput(output) {

test/integration/server.js

@@ -129,6 +129,13 @@ describe("Integration of lib/factorio/server", function() {
 				let result = await server.sendRcon("/sc rcon.print('success')");
 				assert.equal(result, 'success\n');
 			});
+			it("throws on non-empty response when enabled", async function() {
+				slowTest(this);
+				await assert.rejects(
+					server.sendRcon("/sc rcon.print('fail')", true),
+					new Error('Expected empty response but got "fail\n"')
+				);
+			});
 		});
 
 		describe(".stop()", function() {