middleware: Send got_request_exception signal for JSON 500 errors.

This is ordinarily emitted by Django at
  https://github.com/django/django/blob/4.2.6/django/core/handlers/exception.py#L139
and received by Sentry at
  https://github.com/getsentry/sentry-python/blob/1.31.0/sentry_sdk/integrations/django/__init__.py#L166

Signed-off-by: Anders Kaseorg <anders@zulip.com>

zerver/openapi/openapi.py

@@ -373,9 +373,6 @@ def validate_against_openapi_schema(
     # This first set of checks are primarily training wheels that we
     # hope to eliminate over time as we improve our API documentation.
 
-    # No 500 responses have been documented, so skip them
-    if status_code.startswith("5"):
-        return False
     if path not in openapi_spec.openapi()["paths"]:
         endpoint = find_openapi_endpoint(path)
         # If it doesn't match it hasn't been documented yet.