Follow Exception Handling Design Similar To Conductor

[jas34]

While working on the PR with @vivianzheng404 on the PR - #32 we realised that we should opt for the exception handling design which is similar to conductor version v3.13.5. This ticket is being opened to implement after release-3.0.0.
Refer conversation - https://github.com/jas34/scheduledwf/pull/32/files#r1251737069

[vivianzheng404]

2 more things about error handling

• Add error handling when GET a non-exist scheduled workflow
  currently when GET a non-exist scheduled workflow on endpoint /api/scheduling/metadata/scheduleWf/{name}, the response is 200 OK. In the future I expect 404 error code, with response body similar to PUT a non-exist scheduled workflow, such as

{
"status": 404,
"message": "No such workflow found by id: 8666a0e4-e687-452a-b3f4-c6e090323474",
"instance": "vivianzheng-Mac",
"retryable": false
}

• Change error code on endpoints
  Right now when PUT a non-exist scheduled workflow, the error code is 500. However, in the code, it should be 400 INVALID_INPUT. It looks like somehow in the error call stack 400 was overridden/bubbled up to be 500. We should test other error handling endpoints and make sure the error code is correct.