[FEATURE] adding too many requests error (plangrid#120)

* adding too many requestss call

* fix formatting

Author: Fabian

flask_rebar/errors.py

@@ -113,6 +113,10 @@ class UnprocessableEntity(HttpJsonError):
     http_status_code, default_message = 422, "Unprocessable Entity"
 
 
+class TooManyRequests(HttpJsonError):
+    http_status_code, default_message = 429, "Too Many Requests"
+
+
 class InternalError(HttpJsonError):
     http_status_code, default_message = 500, "Internal Server Error"