Log caught exceptions in server installer at debug level
    
    exception() is a helper that logs a traceback at the ERROR
    level. Since the stdout logs at INFO the traceback was going to
    both the log file and stdout. We really only want it in the debug
    log.
    
    https://fedorahosted.org/ipsilon/ticket/143
    
    Signed-off-by: Rob Crittenden <rcritten@redhat.com>
    Reviewed-by: Patrick Uiterwijk <puiterwijk@redhat.com>