Fix error code on clpreauth module failure

Commit 632260bd1fccfb420f0827b59c85c329203eafc9 (ticket #7517) allows
better error reporting for some client pre-authentication failures.
However, it breaks an assumption in the S4U2Self code that such errors
can be recognized by the KRB5_PREAUTH_FAILED error code.  Instead of
passing through the error code reported by the first real preauth
module, wrap that error and return KRB5_PREAUTH_FAILED.

https://github.com/krb5/krb5/commit/560e11dabb63b141df29c54aaa2e120309a1e021
Author: Greg Hudson <ghudson@mit.edu>
Commit: 560e11dabb63b141df29c54aaa2e120309a1e021
Branch: master
 src/lib/krb5/krb/preauth2.c |    8 ++++++--
 1 files changed, 6 insertions(+), 2 deletions(-)