Bubble up errors from webpack run

Author: Vladimir Pouzanov

packages/gatsby/src/commands/build-javascript.js

@@ -11,7 +11,20 @@ module.exports = async program => {
     `build-javascript`
   )
 
-  return new Promise(resolve => {
-    webpack(compilerConfig.resolve()).run(() => resolve())
+  return new Promise((resolve, reject) => {
+    webpack(compilerConfig.resolve()).run((err, stats) => {
+      if (err) {
+        reject(err)
+        return
+      }
+
+      const jsonStats = stats.toJson()
+      if (jsonStats.errors && jsonStats.errors.length > 0) {
+        reject(jsonStats.errors[0])
+        return
+      }
+
+      resolve()
+    })
   })
 }