Report receiver: fix a race in its tests, and log Gitea's reasons

The first test checked that a report was no longer pending as soon as
its log reached the fake Gitea, a moment before the receiver marks it
filed, so it could fail; wait for the mark instead. The tests now close
their servers and error responses.

When Gitea refuses an issue or attachment, the receiver closes the reply
and logs Gitea's reason, such as a token without the right permission.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

server/report-receiver/receiver.py

@@ -191,8 +191,14 @@ class Gitea:
             "Content-Type": content_type,
             "Accept": "application/json",
         })
-        with urllib.request.urlopen(request, timeout=self.timeout) as response:
-            return json.load(response)
+        try:
+            with urllib.request.urlopen(request, timeout=self.timeout) as response:
+                return json.load(response)
+        except urllib.error.HTTPError as error:
+            # Gitea's reason, e.g. a token without the right permission, for the journal
+            with error:
+                detail = error.read(500).decode("utf-8", errors="replace").strip()
+            raise OSError(f"Gitea answered {error.code}: {detail or error.reason}") from None
 
     def create_issue(self, title, body):
         """The new issue's number."""