harness-cdp: when the harness did not run, print the page's uncaught exceptions (a module that failed to load, say)

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Dhu3iHseRewQ2WpBSVLokG

tools/harness-cdp.py

@@ -20,6 +20,7 @@ class WS:
         assert b" 101 " in buf.split(b"\r\n")[0], buf[:200]
         self.buf = buf.split(b"\r\n\r\n", 1)[1]
         self.n = 0
+        self.errors = []                                   # uncaught exceptions the page reported, for the diagnosis
 
     def _read(self, n):
         while len(self.buf) < n: self.buf += self.s.recv(65536)
@@ -52,6 +53,9 @@ class WS:
         self.send({"id": self.n, "method": method, "params": params})
         while True:
             m = self.recv()
+            if m.get("method") == "Runtime.exceptionThrown":
+                d = m["params"]["exceptionDetails"]; e = d.get("exception") or {}
+                self.errors.append(f"{e.get('description') or d.get('text')} @{d.get('url', '')}:{d.get('lineNumber', '')}")
             if m.get("id") == self.n: return m.get("result", {})
 
 
@@ -79,7 +83,7 @@ def main():
             r = ws.call("Runtime.evaluate", expression="(document.getElementById('diag')||{}).textContent||''", returnByValue=True)
             text = r.get("result", {}).get("value", "") or ""
             if "\nEND" in text or text.startswith("END") or text.startswith("TOPERR"): break
-        print(text if text else "no diag: the harness did not run")
+        print(text if text else "no diag: the harness did not run" + "".join("\n  " + e.splitlines()[0] for e in ws.errors))
     finally:
         p.kill()
         subprocess.run(["rm", "-rf", prof])