Improve error message for uv init already init-ed (#7198)

Someone in
https://discord.com/channels/1039017663004942429/1207998321562619954/1282411049106145400
found this confusing

Could maybe improve further, e.g. what the user should do next might
depend if there are [project] or [tool.uv] sections

---------

Co-authored-by: Charlie Marsh <crmarsh416@gmail.com>

crates/uv/src/commands/project/init.rs

@@ -50,7 +50,7 @@ pub(crate) async fn init(
     if path.join("pyproject.toml").exists() {
         let path = std::path::absolute(&path).unwrap_or_else(|_| path.simplified().to_path_buf());
         anyhow::bail!(
-            "Project is already initialized in `{}`",
+            "Project is already initialized in `{}` (`pyproject.toml` file exists)",
             path.display().cyan()
         );
     }

crates/uv/tests/init.rs

@@ -1025,7 +1025,7 @@ fn init_normalized_names() -> Result<()> {
     ----- stdout -----
 
     ----- stderr -----
-    error: Project is already initialized in `[TEMP_DIR]/foo-bar`
+    error: Project is already initialized in `[TEMP_DIR]/foo-bar` (`pyproject.toml` file exists)
     "###);
 
     let child = context.temp_dir.child("foo-bar");