Improve stack trace of exceptions from read(Characteristic)

[spkersten]

Dematerialize exceptions inside readCharacteristic instead of in some stream transformation so the stack trace is the one for the call to readCharacteristic (ending in the user's code that initiated the read that failed) instead of some generic list of stream handling methods.

After:

flutter: Exception: GenericFailure<CharacteristicValueUpdateError>(code: CharacteristicValueUpdateError.unknown, message: "boom")
flutter: #0      Result.dematerialize.<anonymous closure> (package:reactive_ble_platform_interface/src/model/result.dart:22:13)
flutter: #1      Result.iif (package:reactive_ble_platform_interface/src/model/result.dart:34:21)
flutter: #2      Result.dematerialize (package:reactive_ble_platform_interface/src/model/result.dart:15:28)
flutter: #3      ConnectedDeviceOperationImpl.readCharacteristic (package:flutter_reactive_ble/src/connected_device_operation.dart:62:19)
flutter: <asynchronous suspension>
flutter: #4      MyBleClient._bindConnectedDevice (package:ble_client/src/example/example.dart:766:5)

Before:

flutter: Exception: GenericFailure<CharacteristicValueUpdateError>(code: CharacteristicValueUpdateError.unknown, message: "boom")
flutter: #0      Result.dematerialize.<anonymous closure> (package:reactive_ble_platform_interface/src/model/result.dart:22:13)
flutter: #1      Result.iif (package:reactive_ble_platform_interface/src/model/result.dart:34:21)
flutter: #2      Result.dematerialize (package:reactive_ble_platform_interface/src/model/result.dart:15:28)
flutter: #3      ConnectedDeviceOperationImpl.readCharacteristic.<anonymous closure> (package:flutter_reactive_ble/src/connected_device_operation.dart:51:172)
flutter: #4      _MapStream._handleData (dart:async/stream_pipe.dart:211:31)
flutter: #5      _ForwardingStreamSubscription._handleData (dart:async/stream_pipe.dart:151:13)
flutter: #6      _rootRunUnary (dart:async/zone.dart:1422:47)
flutter: #7      _CustomZone.runUnary (dart:async/zone.dart:1324:19)
flutter: #8      _CustomZone.runUnaryGuarded (dart:async/zone.dart:1233:7)
flutter: #9      _BufferingStreamSubscription._sendData (dart:async/stream_impl.dart:366:11)
flutter: #10     _BufferingStreamSubscription._add (dart:async/stream_impl.dart:297:7)
flutter: #11     _ForwardingStreamSubscription._add (dart:async/stream_pipe.dart:121:11)
flutter: #12     _WhereStream._handleData (dart:async/stream_pipe.dart:193:12)
flutter: #13     _ForwardingStreamSubscription._handleData (dart:async/stream_pipe.dart:151:13)
flutter: #14     _rootRunUnary (dart:async/zone.dart:1422:47)
flutter: #15     _CustomZone.runUnary (dart:async/zone.dart:1324:19)
flutter: #16     _CustomZone.runUnaryGuarded (dart:async/zone.dart:1233:7)
flutter: #17     _BufferingStreamSubscription._sendData (dart:async/stream_impl.dart:366:11)
flutter: #18     _BufferingStreamSubscription._add (dart:async/stream_impl.dart:297:7)
flutter: #19     _ForwardingStreamSubscription._add (dart:async/stream_pipe.dart:121:11)