feat(@angular/cli): improve error messages for apps that do not have 'main' defined

Author: vsavkin

packages/@angular/cli/tasks/build.ts

@@ -26,6 +26,9 @@ export default Task.extend({
     if (config.project && config.project.ejected) {
       throw new SilentError('An ejected project cannot use the build command anymore.');
     }
+    if (! app.main) {
+      throw new SilentError(`An app without 'main' cannot use the build command.`);
+    }
     if (runTaskOptions.deleteOutputPath) {
       fs.removeSync(path.resolve(this.project.root, outputPath));
     }

packages/@angular/cli/tasks/test.ts

@@ -21,6 +21,9 @@ export default Task.extend({
     if (appConfig.platform === 'server') {
       throw new SilentError('ng test for platform server applications is coming soon!');
     }
+    if (! appConfig.main) {
+      throw new SilentError(`An app without 'main' cannot use the test command.`);
+    }
 
     return new Promise((resolve) => {
       const karma = requireProjectModule(projectRoot, 'karma');