machine-exec: expose exit code on failure

[benoitf]

Is your enhancement related to a problem? Please describe

Now, on failure, we've a JSON-RPC method onExecError that is saying there was an error and we have a stack attribute containing the message like : 'this process has exited with error code 123`

export interface ExecErrorEvent {
    id: number;
    stack: string;
}

but we don't have the exitCode as a field

Describe the solution you'd like

Provides the exitCode as part of a new attribute so there is nothing to parse on client side to know the exit code.

Describe alternatives you've considered

N/A

Additional context

it seems exit code can be grabbed for kubectl tool:
https://github.com/kubernetes/kubernetes/pull/26541/files

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