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>