fix(serve): if a build error occurs, return config if non-fatal

The promise chain is restored by a .catch(), so we need to continue the
chain with the appropriate resolved value.

This should fix ionic-team/ionic-cli#2247

src/serve.ts

@@ -68,6 +68,7 @@ export function serve(context: BuildContext) {
         throw err;
       } else {
         onReady(config, context);
+        return config;
       }
     });
 }