Fix https://issues.apache.org/bugzilla/show_bug.cgi?id=56523
When using SPNEGO authentication, log the exceptions associated with failed user logins at debug level rather than error level.

git-svn-id: https://svn.apache.org/repos/asf/tomcat/trunk@1594730 13f79535-47bb-0310-9956-ffa450edef68