Prevent panic when dir {project_name} exists already.

[iamnbutler]

Issue

1. cargo run -q || cargo run -q -- --name "my-app"
2. cargo run -q || cargo run -q -- --name "my-app" (again)
3. 💥 Panic due to dir already existing:

   thread 'main' panicked at src/main.rs:24:27:
   Failed to create src directory: Os { code: 17, kind: AlreadyExists, message: "File exists" }

Fix

Now we check if the given dir already exists and return a message instead of panicking.

1. cargo run -q || cargo run -q -- --name "my-app"
2. cargo run -q || cargo run -q -- --name "my-app" (again)
3. 'gpui_app' already exists. || 'my-app' already exists.