Raise a werkzeug exception when the user is over their rate limit

Returning a body and status code means that this response will be
sent directly to the client as text/plain.
Using an exception allows us to use flask's request error handling to
format the error using the upstream app's configuration so that we can,
for example, format the error as JSON for API responses and HTML for
website responses.

brainzutils/ratelimit.py

@@ -6,7 +6,10 @@
 #
 import time
 from functools import update_wrapper
+
 from flask import request, g
+from werkzeug.exceptions import TooManyRequests
+
 from brainzutils import cache
 
 # g key for the timeout when limits must be refreshed from cache
@@ -155,8 +158,9 @@ def on_over_limit(limit):
     ''' 
         Set a nice and readable error message for over the limit requests.
     '''
-    return 'You have exceeded your rate limit. See the X-RateLimit-* response headers for more ' \
-           'information on your current rate limit.\n', 429
+    raise TooManyRequests(
+        'You have exceeded your rate limit. See the X-RateLimit-* response headers for more ' \
+        'information on your current rate limit.')
 
 
 def check_limit_freshness():