ssh remoting: Show error message if project path does not exist (#18343)

This now shows an error message if you try open a project over SSH that
doesn't exist. If it's a possible file-path though, it acts like Zed's
`cli` and opens the file so that it can be created.

- Works: `cargo run ssh://127.0.0.1/~/folder-exists/file-does-not-exist`
— this will open `file-does-not-exist`
- Shows error: `cargo run
ssh://127.0.0.1/~/folder-does-not-exist/file-does-not-exist` — this will
show an error

Release Notes:

- N/A

Co-authored-by: Bennet <bennet@zed.dev>
Co-authored-by: Conrad <conrad@zed.dev>