fix: show message for stopping webpack only when it has been started (#821)

Currently the message `Stopping webpack watch` is printed whenever CLI executes `after-watch` hooks, i.e. when user runs `tns run <platform>` and sends Ctrl+C, the message is shown. However, we have not started any webpack process, so there's nothing to stop.
Print the message only when webpack process had been started. Also print it only when `--log trace` is used.

Author: rosen-vladimirov

Diff for: lib/after-watch.js

@@ -2,8 +2,7 @@ const { stopWebpackCompiler } = require('./compiler');
 const { removeListener } = require("./utils");
 
 module.exports = function($logger, $liveSyncService) {
-    $logger.info("Stopping webpack watch");
-    stopWebpackCompiler();
+    stopWebpackCompiler($logger);
     removeListener($liveSyncService, "liveSyncStopped");
     removeListener(process, "exit");
 }

Diff for: lib/before-watch.js

@@ -10,11 +10,11 @@ module.exports = function ($logger, $liveSyncService, $devicesService, hookArgs)
                 Object.keys(webpackProcesses).forEach(platform => {
                     const devices = $devicesService.getDevicesForPlatform(platform);
                     if (!devices || !devices.length) {
-                        stopWebpackCompiler(platform);
+                        stopWebpackCompiler($logger, platform);
                     }
                 });
             });
-            addListener(process, "exit", stopWebpackCompiler);
+            addListener(process, "exit", () => stopWebpackCompiler($logger));
 
             const platforms = hookArgs.config.platforms;
             return Promise.all(platforms.map(platform => {

Diff for: lib/compiler.js

@@ -123,11 +123,11 @@ exports.runWebpackCompiler = function runWebpackCompiler(config, $projectData, $
     }
 }
 
-exports.stopWebpackCompiler = function stopWebpackCompiler(platform) {
+exports.stopWebpackCompiler = function stopWebpackCompiler($logger, platform) {
     if (platform) {
-        stopWebpackForPlatform(platform);
+        stopWebpackForPlatform($logger, platform);
     } else {
-        Object.keys(webpackProcesses).forEach(platform => stopWebpackForPlatform(platform));
+        Object.keys(webpackProcesses).forEach(platform => stopWebpackForPlatform($logger, platform));
     }
 }
 
@@ -171,9 +171,11 @@ function logSnapshotWarningMessage($logger) {
     }
 }
 
-function stopWebpackForPlatform(platform) {
+function stopWebpackForPlatform($logger, platform) {
+    $logger.trace(`Stopping webpack watch for platform ${platform}.`);
     const webpackProcess = webpackProcesses[platform];
     webpackProcess.kill("SIGINT");
+
     delete webpackProcesses[platform];
 }