NIFIREG-131 Surface auth failure details

[kevdoran]

Adds logging of root cause for exceptions passed to
AuthenticationEntryPoint.

AuthenticationEntryPoint writes exception message to response body.

[kevdoran]

This change results in the following responses when testing a composite file+ldap authorizer and accessing and endpoint that requires authentication:

---

No credentials:

HTTP/1.1 401 Unauthorized
...

An Authentication object was not found in the SecurityContext. Contact the system administrator.

Bad credentials:

HTTP/1.1 401 Unauthorized
...

Unable to validate the access token. Contact the system administrator.

Untrusted Proxy:

HTTP/1.1 403 Forbidden
...

Untrusted proxy [CN=user1, OU=nifi]. Contact the system administrator.

---

Logging for the root cause exception has also been added to the AuthenticationEntryPoint bean.

[bbende]

Reviewing...

[bbende]

+1 Verified I got back the untrusted proxy error on the client side, and also saw it in the logs on registry server side, going to merge, thanks!