Switch error output to JSON

[rhamzeh]

Instead of errors like:

$ ./fga store get --store-id invalid-id

failed to initialize FGA Client due to Configuration.StoreId is not a valid ulid

We'd like to expose errors as json, and have the same display as normal output

[Siddhant-K-code]

@rhamzeh Do you have decided on any particular output spec? It could be something like this if it's not required to be nested:

{
  "error": {
    "code": "<ERROR_CODE>",
    "message": "<HUMAN_READABLE_MESSAGE>",
    "details": "<OPTIONAL_DETAILED_MESSAGE>"
  }
}