[Proposal] Improve toString() of InterceptorState

Request Statement

When Firebase Crashlytics gets an error report from code within a request interceptor,
the information from the log is very limited.

For example:

// Crashlytics report header
DioMixin.request
io.flutter.plugins.firebase.crashlytics.FlutterError - Instance of 'InterceptorState<DioException>'

// Raw stacktrace
      Fatal Exception: io.flutter.plugins.firebase.crashlytics.FlutterError: Instance of 'InterceptorState<DioException>'
       at DioMixin.request(dio_mixin.dart:350)
       at DioMixin.get(dio_mixin.dart:64)
       at HttpChatRepository.getChatMessages(http_chat_repository.dart:12)
       at ChatNotifier.fetchMessages(chat_notifier.dart:38)
       at PollingManager._queue(polling_manager.dart:19)
       at PollingManager.startPolling.<fn>(polling_manager.dart:32)

Solution Brainstorm

Instead of having InterceptorState<T> use the default toString() implementation,
which is Instance of 'InterceptorState<DioException>',
provide the error details of the underlying DioException in the toString() implementation,
by implementing toString() in

dio/dio/lib/src/interceptor.dart

Line 14 in f87a095

class InterceptorState<T> {

and providing the details of the DioException in the toString() implementation.