Improve error message if a connector with duplicate name is added to the same subgraph

### Component

### What is the current behaviour?

```sh
✦ ❯ ddn connector init my_pg --subgraph sql_subgraph/subgraph.yaml --hub-connector hasura/sqlserver --configure-port 8085 --add-to-compose-file compose.yaml
5:30PM INF Checking for update...
5:30PM INF ddn cli is up to date version={"CLI":"v1.0.0","CLISemver":"1.0.0"}
5:30PM ERR Failed: directory "sql_subgraph/connector/my_pg" is not empty
```
Users might misunderstand this error and just remove the `my_pg` folder 😅 to make the command work.

### What is the expected behaviour?

Throw an error message something like:

```
Connector named `my_pg` already exists in the subgraph `sql_subgraph`
```

### How to reproduce the issue?

1.
2.
3.

### Screenshots or Screencast

### Please provide any traces or logs that could help here.

### Any possible solutions/workarounds you're aware of?

### Keywords

Start by reproducing the `ddn connector init my_pg --subgraph sql_subgraph/subgraph.yaml` command and trace the CLI entry point that reports `directory ... is not empty`. Confirm that the duplicate connector name is detected before directory creation, and that the final error identifies the connector and subgraph instead of suggesting only a non-empty directory.

Written by the indexing model from the issue text.