Verify credentials once provided to propose testing other credentials

```bash
cargo run -- --login password --credentials . --store .
```

Output:

```
Compiling matrix-commander v0.11.0 (/home/ubuntu/matrix-commander-rs)
Finished `dev` profile [unoptimized + debuginfo] target(s) in 18.36s
Running `target/debug/matrix-commander-rs --login password --credentials . --store .`
Enter your Matrix homeserver (e.g. https://some.homeserver.org): https://matrix.org
Enter your full Matrix username (e.g. @john:some.homeserver.org): @overclock3000:matrix.org
Enter Matrix password for this user:
Enter your desired name for the Matrix device that is going to be created for you (e.g. matrix-commander-rs): matrix-commander-rs
Enter name of one of your Matrix rooms that you want to use as default room (e.g. !someRoomId:some.homeserver.org): !sNARMdEsFZERaQAJzl:matrix.org
2026-07-04T15:41:20.727662Z ERROR matrix_sdk::http_client: Error while sending request: Api(Server(MatrixError(Error { status_code: 403, body: Standard(StandardErrorBody { kind: Forbidden, message: "Invalid username/password" }) })))
2026-07-04T15:41:20.728094Z ERROR matrix_commander_rs::mclient: Error: the server returned an error: [403 / M_FORBIDDEN] Invalid username/password
2026-07-04T15:41:20.729048Z ERROR matrix_commander_rs: Login to server failed or credentials information could not be written to disk. Check your arguments and try --login again. Reported error is: LoginFailed
Error: LoginFailed
```

The error does not state if username exists it seems, can it be figured out even with a wrong password? If so, then could request only password.