fix(@ngtools/webpack): show error stack on plugin

Plugin errors do not show the stack, just the message. This makes it harder to debug errors as the single line message blends into loader errors.

packages/@ngtools/webpack/src/plugin.ts

@@ -579,7 +579,7 @@ export class AotPlugin implements Tapable {
 
         cb();
       }, (err: any) => {
-        compilation.errors.push(err);
+        compilation.errors.push(err.stack);
         cb();
       });
   }