tests: Add golden tests for error conditions

tests/golden/error-empty.yml

+input:
+http_status: 500
+
+exit_code: 1
+output: |
+  Last change: 0 seconds ago
+  Server error '500 Internal Server Error' for url 
+  'http://localhost:5000/testbook'
+  For more information check: https://httpstatuses.com/500

tests/golden/error-json.yml

+input:
+  error: SomethingWentWrongError
+  msg: Alles kaputt
+http_status: 400
+
+exit_code: 1
+output: |
+  Last change: 0 seconds ago
+  Client error '400 Bad Request' for url 'http://localhost:5000/testbook'
+  For more information check: https://httpstatuses.com/400
+  Error: SomethingWentWrongError
+  Message: Alles kaputt
+  💥 Submission failed! (2022-10-12 13:37:00)

tests/test_cli.py

@@ -140,12 +140,20 @@ def test_golden(
     time_machine: TimeMachineFixture,
     nb_arg: str,
 ) -> None:
-    httpx_mock.add_response(json=golden["input"])  # FIXME add URL?
+    httpx_mock.add_response(  # FIXME add URL?
+        method="POST",
+        json=golden["input"],
+        status_code=golden.get("http_status", 200),
+    )
+
     delay = golden.get("delay", 0)
     time_machine.move_to(FROZEN_TIME + datetime.timedelta(seconds=delay))
 
     result = runner.invoke(cli.app, [nb_arg])
     print(result.stdout)
 
-    assert result.exit_code == 0
+    # FIXME pytest-golden should probably support a fallback for .get, or at least
+    # __contains__
+    exit_code = 0 if golden.get("exit_code") is None else golden.out["exit_code"]
+    assert result.exit_code == exit_code
     assert result.stdout == golden.out["output"]