Improve error messages on extension loading (#42266)

This pull request improves error message when extension loading goes
wrong.

Before:

```
2025-11-08T21:16:02+08:00 ERROR [extension_host::extension_host] failed to load arkts extension.toml

Caused by:
    No such file or directory (os error 2)
```

Now:

```
2025-11-08T22:57:00+08:00 ERROR [extension_host::extension_host] failed to load arkts extension.toml, "/Users/user_name_placeholder/Library/Application Support/Zed/extensions/installed/arkts/extension.toml"

Caused by:
    No such file or directory (os error 2)

```

Release Notes:

- N/A

crates/extension/src/extension_manifest.rs

@@ -267,10 +267,9 @@ impl ExtensionManifest {
 
         let mut extension_manifest_path = extension_dir.join("extension.json");
         if fs.is_file(&extension_manifest_path).await {
-            let manifest_content = fs
-                .load(&extension_manifest_path)
-                .await
-                .with_context(|| format!("failed to load {extension_name} extension.json"))?;
+            let manifest_content = fs.load(&extension_manifest_path).await.with_context(|| {
+                format!("loading {extension_name} extension.json, {extension_manifest_path:?}")
+            })?;
             let manifest_json = serde_json::from_str::<OldExtensionManifest>(&manifest_content)
                 .with_context(|| {
                     format!("invalid extension.json for extension {extension_name}")
@@ -279,10 +278,9 @@ impl ExtensionManifest {
             Ok(manifest_from_old_manifest(manifest_json, extension_name))
         } else {
             extension_manifest_path.set_extension("toml");
-            let manifest_content = fs
-                .load(&extension_manifest_path)
-                .await
-                .with_context(|| format!("failed to load {extension_name} extension.toml"))?;
+            let manifest_content = fs.load(&extension_manifest_path).await.with_context(|| {
+                format!("loading {extension_name} extension.toml, {extension_manifest_path:?}")
+            })?;
             toml::from_str(&manifest_content).map_err(|err| {
                 anyhow!("Invalid extension.toml for extension {extension_name}:\n{err}")
             })