refactor(@angular-devkit/core): UnknownException should still extend Base

Author: hansl

etc/api/angular_devkit/core/src/_golden-api.d.ts

@@ -979,7 +979,7 @@ export declare class UnimplementedException extends BaseException {
     constructor();
 }
 
-export declare class UnknownException extends Error {
+export declare class UnknownException extends BaseException {
     constructor(message: string);
 }
 

packages/angular_devkit/core/src/exception/exception.ts

@@ -13,7 +13,7 @@ export class BaseException extends Error {
 }
 
 
-export class UnknownException extends Error {
+export class UnknownException extends BaseException {
   constructor(message: string) { super(message); }
 }