Fix 500 error return for schedule already exists

[longquanzheng]

{"errors":[{"message":"[500 Internal Server Error] during [POST] to [http://localhost:8801/api/v1/workflow/start] [DefaultApi#apiV1WorkflowStartPost(WorkflowStartRequest)]: [{\"detail\":\"schedule with this ID is already registered\",\"subStatus\":\"UNCATEGORIZED_SUB_STATUS\"}]","code":"500","stackTrace":"feign.RetryableException: [500 Internal Server Error] during [POST] to [http://localhost:8801/api/v1/workflow/start] [DefaultApi#apiV1WorkflowStartPost(WorkflowStartRequest)]: [{\"detail\":\"schedule with this ID is already registered\",\"subStatus\":\"UNCATEGORIZED_SUB_STATUS\"}]\n\tat io.iworkflow.core.ServerErrorRetryDecoder.decode(ServerErrorRetryDecoder.java:23)\n\tat feign.AsyncResponseHandler.handleResponse(AsyncResponseHandler.java:98)\n\tat feign.SynchronousMethodHandler.executeAndDecode(SynchronousMethodHandler.java:141)\n\tat feign.SynchronousMethodHandler.invoke(SynchronousMethodHandler.java:91)\n\tat feign.ReflectiveFeign$FeignInvocationHandler.invoke(ReflectiveFeign.java:100)\n\tat jdk.proxy2/jdk.proxy2.$Proxy110.apiV1WorkflowStartPost(Unknown Source)\n\tat io.iworkflow.core.UnregisteredClient.startWorkflow(UnregisteredClient.java:181)\n\tat io.iworkflow.core.Client.startWorkflow(Client.java:180)\n\tat io.iworkflow.core.Client.startWorkflow(Client.java:111)\n\tat com.indeed.iwf.samples.controllers.ManualBasicController.sta..."}],"status":"INTERNAL_SERVER_ERROR"}