fix: stabilize web loading and paper rendering

web/ocs-parse-worker.js

@@ -3,9 +3,16 @@ import init, { parse_document } from "./worker_pkg/ocs_web_worker.js";
 const ready = init();
 
 self.onmessage = async ({ data }) => {
+  let stage = "initialize worker";
   try {
     await ready;
-    const encoded = parse_document(data.name, new Uint8Array(data.bytes));
+    const encoded = parse_document(
+      data.name,
+      new Uint8Array(data.bytes),
+      (next) => {
+        stage = next;
+      },
+    );
     // wasm-bindgen returns a view into WebAssembly.Memory. Copy to a standalone
     // ArrayBuffer before transferring it, otherwise the worker's wasm memory
     // itself would be detached.
@@ -14,7 +21,9 @@ self.onmessage = async ({ data }) => {
   } catch (error) {
     self.postMessage({
       ok: false,
-      error: error instanceof Error ? error.message : String(error),
+      error: `${stage}: ${
+        error instanceof Error ? error.stack || error.message : String(error)
+      }`,
     });
   }
 };