Fix TS typings for KeyErrorEvent and KeySessionCreatedEvent error (#4441

)

`KeyErrorEvent` is triggered with `error` (`DashJSError`) instead of `data`.

`KeySystemSelectedEvent` has incorrect `error` type:  it's `DashJSError`
as well instead of `string`.

As a result, `KeyError` type becomes redundant and can be removed.

index.d.ts

@@ -1896,16 +1896,9 @@ declare namespace dashjs {
         data: object
     }
 
-    export class KeyError {
-        constructor(sessionToken: SessionToken, errorString: string);
-
-        sessionToken: SessionToken;
-        error: string;
-    }
-
     export interface KeyErrorEvent extends Event {
         type: MediaPlayerEvents['KEY_ERROR'];
-        data: KeyError;
+        error: DashJSError;
     }
 
     export class KeyMessage {
@@ -1943,7 +1936,7 @@ declare namespace dashjs {
     export interface KeySystemSelectedEvent extends Event {
         type: MediaPlayerEvents['KEY_SYSTEM_SELECTED'];
         data: object | null;
-        error?: string;
+        error?: DashJSError;
     }
 
     export interface LicenseRequestCompleteEvent extends Event {