In the event that the Content Event Webhook External Event action callout to
the receiver sample application does not have the expected HMAC credential,
HMACAuthenticationFilter will throw a
HMACSecurityException. This
class handles mapping that exception to a
UNAUTHORIZED response code.
Note that handling for other uncaught exceptions can be added to this class,
or the servlet can handle the exception and return a response. It is up to
the implementor to decide which approach to take with every possible
exception.