Create parent directories for each source file.

On some platforms, fs::write won't create parent
direcotries and will fail if the parent directories
don't exist. Now we make the parent directories
before writing. This also adds context to write errors
indicating what file didn't get written.

recapnc/src/lib.rs

@@ -413,7 +413,12 @@ pub fn generate_from_request(request: &ReaderOf<CodeGeneratorRequest>, out: impl
             .context("parsing generated file")?;
         let printable = prettyplease::unparse(&parsed);
 
-        fs::write(out.join(Path::new(&root.path)), printable)?;
+        let out_path = out.join(Path::new(&root.path));
+        if let Some(parent) = out_path.parent() {
+            let _ = fs::create_dir_all(parent);
+        }
+        fs::write(&out_path, printable)
+            .with_context(|| format!("failed to write to path: {}", out_path.display()))?;
 
         root_mod.files.push(root);
     }