extension_cli: Don't propagate errors caused by trying to read Cargo.toml (#9641)

This PR fixes an issue where the extension CLI would error if a
`Cargo.toml` didn't exist when we were trying to check for its
existence. Since we're just checking if it exists for the purposes of
detecting a Rust extension, we can safely ignore the errors.

Also improved the logging/error handling in a few spots to make other
errors easier to troubleshoot in the future.

Release Notes:

- N/A

crates/extension_cli/src/main.rs

@@ -54,9 +54,11 @@ async fn main() -> Result<()> {
         args.output_dir
     };
 
+    log::info!("loading extension manifest");
     let mut manifest = ExtensionStore::load_extension_manifest(fs.clone(), &extension_path).await?;
     populate_default_paths(&mut manifest, &extension_path)?;
 
+    log::info!("compiling extension");
     let builder = ExtensionBuilder::new(scratch_dir);
     builder
         .compile_extension(
@@ -257,7 +259,7 @@ fn test_languages(
             Some(
                 grammars
                     .get(name.as_ref())
-                    .ok_or_else(|| anyhow!("language"))?,
+                    .ok_or_else(|| anyhow!("grammar not found: '{name}'"))?,
             )
         } else {
             None