Explain: Improve error reporting in Amp backend.

Author: mark-undoio

explain/amp_agent.py

@@ -126,7 +126,7 @@ async def ask(self, question: str, port: int, tools: list[str]) -> str:
             assert amp_start_thread.stdout and amp_start_thread.stderr
 
             stdout_bytes, stderr_bytes = await amp_start_thread.communicate()
-            assert not stderr_bytes
+            assert not stderr_bytes, stderr_bytes
 
             self._thread_id = stdout_bytes.decode("utf-8").rstrip()