feature(webpack): Propagate errors from webpack stats (#871)

* Propagate errors from webpack stats

Prior to this patch, the script would only raise errors raised directly
by webpack and skip errors raised from the compilation. This is contrary
to what webpack suggests in terms of error handling[1].

This patch makes it so that any compilation errors will cause ionic to
also abort the build, while any compilation warnings will log a warning
through ionic's Logger.

* Update webpack.ts

src/webpack.ts

@@ -140,8 +140,20 @@ export function runWebpackFullBuild(config: WebpackConfig) {
     const callback = (err: Error, stats: any) => {
       if (err) {
         reject(new BuildError(err));
-      } else {
-        resolve(stats);
+      }
+      else {
+        const info = stats.toJson();
+
+        if (stats.hasErrors()) {
+          reject(new BuildError(info.errors));
+        }
+        else if (stats.hasWarnings()) {
+          Logger.debug(info.warnings)
+          resolve(stats);
+        }
+        else {
+          resolve(stats);
+        }
       }
     };
     const compiler = webpackApi(config);