python: Migrate most json_error => JsonableError.

JsonableError has two major benefits over json_error:
* It can be raised from anywhere in the codebase, rather than
  being a return value, which is much more convenient for refactoring,
  as one doesn't potentially need to change error handling style when
  extracting a bit of view code to a function.
* It is guaranteed to contain the `code` property, which is helpful
  for API consistency.

Various stragglers are not updated because JsonableError requires
subclassing in order to specify custom data or HTTP status codes.

zerver/webhooks/front/view.py

@@ -4,8 +4,9 @@ from django.http import HttpRequest, HttpResponse
 from django.utils.translation import gettext as _
 
 from zerver.decorator import webhook_view
+from zerver.lib.exceptions import JsonableError
 from zerver.lib.request import REQ, has_request_variables
-from zerver.lib.response import json_error, json_success
+from zerver.lib.response import json_success
 from zerver.lib.webhooks.common import check_send_webhook_message
 from zerver.models import UserProfile
 
@@ -143,7 +144,7 @@ def api_front_webhook(
 
     event = payload["type"]
     if event not in EVENT_FUNCTION_MAPPER:
-        return json_error(_("Unknown webhook request"))
+        raise JsonableError(_("Unknown webhook request"))
 
     topic = payload["conversation"]["id"]
     body = get_body_based_on_event(event)(payload)