Allow Error return customization

[driverpt]

Is your feature request related to a problem? Please describe.
We have the need to customize Error responses, e.g.: Add an Error Code/Status Code. And we feel that it's kind of limiting to only have messages.InvokeResponse_Error{} as Error Return.

Describe the solution you'd like
Is it possible to make this customizable via Custom JSON Serialization ?

Describe alternatives you've considered
N/A

Additional context
N/A

[bmoffatt]

integrations that call Lambdas have had years to ossify the assumption that an error json will look something like this:

{
...
  "errorMessage": ...,
...
}

so the approach for this probably shouldn't be as simple as delegating json.Marshal, that feels like a footgun for a bunch of users if errorMessage could accidentally go missing. Somehow the messages.InvokeResponse_Error{} and your custom serialization result would have to be merged. Or perhaps the custom serialization result nested inside like:

type InvokeResponse_Error struct {
	Message    string                             `json:"errorMessage"`
	Type       string                             `json:"errorType"`
	StackTrace []*InvokeResponse_Error_StackFrame `json:"stackTrace,omitempty"`
	ShouldExit bool                               `json:"-"`
	// new field, populated with the result of `json.Marshal` if the handler error has a `MarshalJSON` receiver defined
	Custom json.RawMessage                        `json:"custom,omitempty"`
}

/cc @carlzogh

If we went this this approach, we'd also wanna check that it behaves with the go1.x RPC client.

---

@driverpt Are you running your functions with go1.x, provided, or provided.al2? If your custom fields appeared nested in the existing error json, would that still fit your use case?

[driverpt]

Our functions are running on provided.al2.
Ideally it would have a json.Marshall(err).

Is it possible to have a Global Flag to tell the handler to use Custom errors?