Improve ipa-replica-prepare DNS check
Currently, verify_fqdn() function raises RuntimeError for every
problem with the hostname. This makes it difficult for tools
like ipa-replica-prepare to behave differently for a subset of
raised errors (for example to be able to create a DNS record for
new replica when verify_fqdn() reports a lookup error).
Implement own exceptions for verify_fqdn() that they can be safely
used to distinguish the error type.
https://fedorahosted.org/freeipa/ticket/1899