api docs: Add page for common error payloads.

We now have a separate page for common error payloads, for example,
the payload for when the client's API key is invalid. All error
payloads that are presented on this page will be tested similarly
to our other non-error sample fixtures.

zerver/lib/api_test_helpers.py

@@ -279,6 +279,13 @@ def update_message(client, message_id):
     assert result['result'] == 'success'
     assert result['raw_content'] == request['content']
 
+def test_invalid_api_key(client_with_invalid_key):
+    # type: (Client) -> None
+    result = client_with_invalid_key.list_subscriptions()
+    fixture = FIXTURES['invalid-api-key']
+    test_against_fixture(result, fixture)
+
+
 TEST_FUNCTIONS = {
     'render-message': render_message,
     'stream-message': stream_message,
@@ -343,5 +350,3 @@ def test_the_api(client):
     test_users(client)
     test_streams(client)
     test_messages(client)
-
-    # print(dir(client))