Troubleshoot and fix /list-workflows and /discovery-executions

[botanical]

GET /list-workflows and GET /discovery-executions with a workflow_execution_id that doesn't exist returns an Internal Server Error

/list-workflows error:

"Unhandled exception: 2 validation errors:\n  {'loc': ('response',), 'msg': 'value is not a valid dict', 'type': 'type_error.dict'}\n  {'loc': ('response',), 'msg': 'value is not a valid dict', 'type': 'type_error.dict'}\n",

/discovery-executions error:

\| Exception: Failed to find dag run id: 9ec62382-deca-436b-8038-b3e835cd837e_be3d675f-2f8b-43c4-904c-d113c04477f5

For the discovery-executions exception, if the dag run id can not be found, we should throw a more helpful error message.