errors: Include request info on error mails for JSON routes too.

When our code raises an exception and Django converts it to a 500
response (in django.core.handlers.exception.handle_uncaught_exception),
it attaches the request to the log record, and we use this in our
AdminNotifyHandler to include data like the user and the URL path
in the error email sent to admins.

On this line, when our code raises an exception but we've decided (in
`TagRequests`) to format any errors as JSON errors, we suppress the
exception so we have to generate the log record ourselves.  Attach the
request here, just like Django does when we let it do the job.

This still isn't an awesome solution, in that there are lots of other
places where we call `logging.error` or `logging.exception` while
inside a request; this just covers one of them.  This is one of the
most common, though, so it's a start.

zerver/middleware.py

@@ -278,7 +278,7 @@ class JsonErrorHandler(MiddlewareMixin):
         if isinstance(exception, JsonableError):
             return json_response_from_error(exception)
         if request.error_format == "JSON":
-            logging.error(traceback.format_exc())
+            logging.error(traceback.format_exc(), extra=dict(request=request))
             return json_error(_("Internal server error"), status=500)
         return None