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.

tools/test-api

@@ -14,7 +14,7 @@ os.chdir(ZULIP_PATH)
 from zulip import Client
 
 from tools.lib.test_server import test_server_running
-from zerver.lib.api_test_helpers import test_the_api
+from zerver.lib.api_test_helpers import test_the_api, test_invalid_api_key
 
 os.environ['DJANGO_SETTINGS_MODULE'] = 'zproject.test_settings'
 django.setup()
@@ -35,4 +35,13 @@ with test_server_running(external_host='zulipdev.com:9981'):
     print("Running API tests...")
     test_the_api(client)
 
+    # Test error payloads
+    client = Client(
+        email=email,
+        api_key='abcedrsdfd',
+        site=site
+    )
+    test_invalid_api_key(client)
+
+
 print("API tests passed!")