Make Continuous installer continuous only during execution phase
    
    `common.Continuous` class is a basis for uninstallers, which should execute
    all the steps regardless of occuring errors. However, we would like the
    installer to raise exceptions and return non-zero exit code during validation
    phase when some preconditions are not met.
    
    Add a separate exception handler which catches exceptions and logs them as
    errors during execution phase only.
    
    https://fedorahosted.org/freeipa/ticket/5725
    
    Reviewed-By: Petr Spacek <pspacek@redhat.com>