Fixes #29

The errors list was not being cleared out after resuming a backup
from a rate limit sleep. When the backup was resumed, the non-empty
errors list caused the backup to quit after the next `retrieve_data`
request.

bin/github-backup

@@ -299,10 +299,10 @@ def get_query_args(query_args=None):
 
 def _get_response(request, auth, template):
     retry_timeout = 3
-    errors = []
     # We'll make requests in a loop so we can
     # delay and retry in the case of rate-limiting
     while True:
+        errors = []
         should_continue = False
         try:
             r = urllib2.urlopen(request)