fixExceptionMessageInGatewayDiagnostics

[xinlian12]

Issue:
for non-cosmos exception, the exceptionMessage is not populated in gatewayDiagnostics. Changed to populate the error message from inner cause as well.

 'gatewayStatisticsList': [{
                'sessionToken': null,
                'operationType': 'Query',
                'resourceType': 'Document',
                'statusCode': 0,
                'subStatusCode': 0,
                'requestCharge': 0.0,
                'requestTimeline': [{
                        'eventName': 'connectionAcquired',
                        'startTimeUTC': '2024-03-05T11:08:00.498624Z',
                        'durationInMilliSecs': 0.055
                    }, {
                        'eventName': 'connectionConfigured',
                        'startTimeUTC': '2024-03-05T11:08:00.498679Z',
                        'durationInMilliSecs': 0.003
                    }, {
                        'eventName': 'requestSent',
                        'startTimeUTC': '2024-03-05T11:08:00.498682Z',
                        'durationInMilliSecs': 0.119
                    }, {
                        'eventName': 'transitTime',
                        'startTimeUTC': '2024-03-05T11:08:00.498801Z',
                        'durationInMilliSecs': 283.278
                    }, {
                        'eventName': 'received',
                        'startTimeUTC': null,
                        'durationInMilliSecs': 0.0
                    }
                ],
                'partitionKeyRangeId': null,
                'responsePayloadSizeInBytes': 0,
                'exceptionResponseHeaders': '{}'
            }
        ],

[FabianMeiswinkel]

LGTM