Add AccessDeniedException to newrelic ignore list

These exceptions are typically user error. However in the case
that a user session has gone stale while in edit mode in the
staff interface the update monitor polling will generate a lot of
AccessDeniedException messages which are a "false alarm" from a
monitoring point of view.
Merge pull request #78 from mark-cooper/newrelic_ignore_access_denied

Add AccessDeniedException to newrelic ignore list