Don't make error data payload extensible

as this makes it too easy for drivers to put data in the wrong section.

Still define the payload type as an interface so that it can be
extended in a future version of the API.

Also don't use a subfield now that non-extensibility makes the format of
the details fields unambiguous.

Author: AndrewFerr

src/ClientWidgetApi.ts

@@ -833,11 +833,11 @@ export class ClientWidgetApi extends EventEmitter {
     }
 
     private handleDriverError(e: unknown, request: IWidgetApiRequest, message: string) {
-        const errorDetails = this.driver.processError(e);
+        const data = this.driver.processError(e);
         this.transport.reply<IWidgetApiErrorResponseData>(request, {
             error: {
                 message,
-                ...errorDetails,
+                ...data,
             },
         });
     }

src/interfaces/IWidgetApiErrorResponse.ts

@@ -34,7 +34,7 @@ export interface IMatrixApiError {
     } & IWidgetApiResponseData; // extensible
 }
 
-export interface IWidgetApiErrorResponseDataDetails extends IWidgetApiResponseData {
+export interface IWidgetApiErrorResponseDataDetails {
     /** Set if the error came from a Matrix API request made by a widget driver */
     matrix_api_error?: IMatrixApiError;  // eslint-disable-line camelcase
 }