Add onlyOnError option

Trigger a notification only on error.

README.md

@@ -78,12 +78,20 @@ new WebpackNotifierPlugin({excludeWarnings: true});
 
 ### Always Notify
 
-Trigger a notification every time.  Call it "noisy-mode".
+Trigger a notification every time. Call it "noisy-mode".
 
 ```js
 new WebpackNotifierPlugin({alwaysNotify: true});
 ```
 
+### Notify on error
+
+Trigger a notification only on error.
+
+```js
+new WebpackNotifierPlugin({onlyOnError: true});
+```
+
 ### Skip Notification on the First Build
 
 Do not notify on the first build.  This allows you to receive notifications on subsequent incremental builds without being notified on the initial build.

index.js

@@ -40,10 +40,10 @@ WebpackNotifierPlugin.prototype.compileMessage = function(stats) {
     if (stats.hasErrors()) {
         error = findFirstDFS(stats.compilation, 'errors');
 
-    } else if (stats.hasWarnings() && !this.options.excludeWarnings) {
+    } else if (stats.hasWarnings() && !this.options.excludeWarnings && !this.options.onlyOnError) {
         error = findFirstDFS(stats.compilation, 'warnings');
 
-    } else if (!this.lastBuildSucceeded || this.options.alwaysNotify) {
+    } else if ((!this.lastBuildSucceeded || this.options.alwaysNotify) && !this.options.onlyOnError) {
         this.lastBuildSucceeded = true;
         return (hasEmoji ? '✅ ' : '') + 'Build successful';
 
@@ -53,7 +53,7 @@ WebpackNotifierPlugin.prototype.compileMessage = function(stats) {
 
     this.lastBuildSucceeded = false;
 
-    var message;
+    var message = '';
     if (error.module && error.module.rawRequest)
         message = error.module.rawRequest + '\n';