fix: handle non-200 http status code

[pudkrong]

Check res.ok before calling res.json().

Found this error

[AWS Parameters and Secrets Lambda Extension] 2023/06/19 22:28:15 INFO ready to serve traffic
[AWS Parameters and Secrets Lambda Extension] 2023/06/19 22:28:15 ERROR GetParameter request encountered an error: operation error SSM: GetParameter, https response error StatusCode: 400, RequestID: 6fa998e8-0d6f-4650-8bcd-2364928299d6, ParameterNotFound: 
2023-06-19T22:28:15.755Z	24f1eec7-128b-4b8f-9994-610b2cb6885b	INFO	{"res":{"status":200}}
2023-06-19T22:28:15.772Z	24f1eec7-128b-4b8f-9994-610b2cb6885b	INFO	{"payload":{"Parameter":{"ARN":"arn:aws:ssm:eu-west-1:812555912232:parameter/muninn/thirdParty/nrfcloud/apiKey","DataType":"text","LastModifiedDate":"2023-06-14T15:52:18.635Z","Name":"/muninn/thirdParty/nrfcloud/apiKey","Selector":null,"SourceResult":null,"Type":"String","Value":"824ae663419469ce05cd7b5634cd4a4cbf2a85a1","Version":2},"ResultMetadata":{}}}
2023-06-19T22:28:15.775Z	24f1eec7-128b-4b8f-9994-610b2cb6885b	INFO	{"res":{"status":400}}
2023-06-19T22:28:15.778Z	24f1eec7-128b-4b8f-9994-610b2cb6885b	ERROR	SyntaxError: Unexpected token a in JSON at position 0
    at JSON.parse (<anonymous>)
    at parseJSONFromBytes (node:internal/deps/undici/undici:6571:19)
    at successSteps (node:internal/deps/undici/undici:6545:27)
    at node:internal/deps/undici/undici:1211:60
    at node:internal/process/task_queues:140:7
    at AsyncResource.runInAsyncScope (node:async_hooks:203:9)
    at AsyncResource.runMicrotask (node:internal/process/task_queues:137:8)
    at process.processTicksAndRejections (node:internal/process/task_queues:95:5)

[coderbyheart]

That change replaces one exception with another, and also breaks the API of the function (should return undefined if parameter is not found). I've updated the implementation to fail more gracefully.