Improve feedback for abstract class instantiation error messages

<!-- BUGS: Please use this template. --> <!-- QUESTIONS: This is not a general support forum! Ask Qs at http://stackoverflow.com/questions/tagged/typescript --> <!-- SUGGESTIONS: See https://github.com/Microsoft/TypeScript-wiki/blob/master/Writing-Good-Design-Proposals.md --> **SUGGESTION** Recently, I was trying to use a library written in ES2015 in an Ionic 2 project, and kept getting a `Cannot instantiate abstract class 'Foo'`. `Foo` is a concrete class that extends abstract class `Bar`, which extends abstract class `Baz`. The problem turned out to be a typo in `Bar.d.ts` (see below), which (I believe) made Typescript think that `Foo` did not implement some abstract methods of `Bar` and/or `Baz` (I'm still not sure which), but it took me a day and half to figure out what was going on. This error message should have more information about why `Foo` is abstract: `Foo is declared as abstract` or `Foo does not implement abstract methods ...` **TypeScript Version:** 2.0.9 **Code** **`Baz.d.ts`** ```ts export as namespace Baz; declare abstract class Baz { abstract baz(): string; } ``` **`Bar.d.ts`** ```ts export as namespace BarTypo; declare abstract class Bar extends Baz { abstract foo(): void; abstract bar(): string; baz(): string; } export { Bar }; ``` **`Foo.d.ts`** ```ts import { Bar } from './bar.d'; export as namespace Foo; declare class Foo extends Bar { constructor(opts?); foo(): void; bar(a: string): string; } export { Foo }; ``` **`otherProject.ts`** ```ts import { Foo } from 'library'; export class Service { foo: Foo; constructor() { this.foo = new Foo({ opt: true }); } } ``` **Expected behavior:** `Error: Cannot instantiate abstract class 'Foo'. It does not implement abstract method(s): baz.` OR `Error: Cannot instantiate abstract class 'Foo'. It does not implement abstract method(s): foo, bar.` OR `Error: Cannot instantiate abstract class 'Foo'. It does not implement abstract method(s): baz (from Baz); foo, bar (from Bar).` **Act…