1103914 - Pulp exceptions no longer log a traceback by default

https://bugzilla.redhat.com/show_bug.cgi?id=1103914

This includes two major changes:

1.  pulp/server/webservices/middleware/exception.py has been changed to log any PulpException derived exception to INFO rather than logging it to ERROR with the traceback. This fixes BZ1103914.

2. The old AuthenticationFailed exception has been replaced by PulpCodedAuthenticationException, which is a subclass of PulpCodedException. However, PulpCodedAuthenticationException includes the old AuthenticationFailed error code, so it's backward compatible.