[FIXED JENKINS-25204] - Handle UsernameNotFoundException in BasicAuthenticationFilter::doFilter()

The change catches the unhandled RuntimeException, which may appear if the system fails to find a user by its name.

The information about the failure will be printed to the logger.
User will get 403 error like he gets when username==null

Signed-off-by: Oleg Nenashev <o.v.nenashev@gmail.com>