fix(6255): Test destination names an address that did not answer as S3

A 4xx with no S3 error body came from something else at that address, most
often a web server on the default port. The message now says so and to check
the scheme and port, beside the raw status. A 5xx without a body keeps its
own reason, since a busy S3 service can answer that way.

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

tests/test_services_backup_s3.py

@@ -78,6 +78,22 @@ def test_a_refusal_says_why(tmp_path):
         asyncio.run(s3.put_file(TARGET, "b.tar", f, transport=transport))
 
 
+def test_a_web_server_at_the_address_is_named_as_not_s3():
+    """No port in the address: the request lands on whatever serves port 80,
+    and its plain 404 says nothing about S3."""
+    page = b"<html><body><h1>404 Not Found</h1></body></html>"
+    transport = httpx.MockTransport(lambda r: httpx.Response(404, content=page))
+    with pytest.raises(s3.S3Error, match=r"HTTP 404: the address answered, but not as S3 — check the scheme and the port"):
+        asyncio.run(s3.list_objects(TARGET, transport=transport))
+
+
+def test_a_server_error_without_a_body_is_not_blamed_on_the_port():
+    transport = httpx.MockTransport(lambda r: httpx.Response(503))
+    with pytest.raises(s3.S3Error, match="HTTP 503: Service Unavailable") as err:
+        asyncio.run(s3.list_objects(TARGET, transport=transport))
+    assert "port" not in str(err.value)
+
+
 def test_unreachable_says_so(tmp_path):
     f = tmp_path / "b.tar"
     f.write_bytes(b"x")