fix(rust): when an item is not found an error is returned

[davide-baldo]

When a show command is executed on any resource and that resource is missing, it'll simply print There are no <type> to show, return a SUCCESS (0) exit code.

$ ockam node show hello
  > There are no nodes to show
$ echo $?
0

This could be misleading, and an error should be printed.
Also, some tests rely on the fact that the shown command returning 0 means the resource exists.

With this patch the resource not found triggers an error instead.

$ target/debug/ockam node show hello


    Error:
    ────────────────────────────────────────────────────────────────────────────────

    There is no node with name hello (origin: Api, kind: NotFound, source location: implementations/rust/ockam/ockam_api/src/cli_state/nodes.rs:311:17)
    Please report this issue, with a copy of your logs, to https://github.com/build-trust/ockam/issues
    
    Error code: OCK500
    version: 0.134.0
    compiled from: d071009f2ce6f9f7c7be1d04e89308abc8c4420a

    If you need help, please email us on [email protected]
    We will promptly help you get unstuck and quickly resolve any problems.

    ────────────────────────────────────────────────────────────────────────────────
$ echo $?
70

While it's nice to return a non-successful exit code, this error message could be too much.