Note that we cannot mark `--rpc-endpoint` flag as required because
it can be taken from config.
Before:
```
Error: can't create N3 client: failed to get network magic: Post "": unsupported protocol scheme ""
```
Now:
```
Error: can't create N3 client: missing endpoint
```
Signed-off-by: Evgenii Stratonikov <evgeniy@nspcc.ru>
