fix(@ngtools/webpack): ensure webpack errors are actually an Error

Author: clydin

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

@@ -691,7 +691,7 @@ export class AngularCompilerPlugin {
         timeEnd('AngularCompilerPlugin._make');
         cb();
       }, (err: any) => {
-        compilation.errors.push(err.stack);
+        compilation.errors.push(err);
         this.pushCompilationErrors(compilation);
         timeEnd('AngularCompilerPlugin._make');
         cb();
@@ -807,7 +807,7 @@ export class AngularCompilerPlugin {
 
         if (errors.length > 0) {
           const message = formatDiagnostics(errors);
-          this._errors.push(message);
+          this._errors.push(new Error(message));
         }
 
         if (warnings.length > 0) {