chore: improve CLI error handling

[blckngm]

Now:

Error: config backends

Caused by:
    0: load validator from /scripts/godwoken-scripts/meta-contract-validator.invalid
    1: No such file or directory (os error 2)

Previously:

thread 'main' panicked at 'run cli: config backends

Caused by:
    No such file or directory (os error 2)', crates/block-producer/src/main.rs:136:21
stack backtrace:
...
[very long stacktrace that is not useful because the panic always happens in main]
...