ipa-replica-install: Move check for existing host before DNS resolution check
The checks for existing host and existing replication agreement
set a flag that caused an exit() if any of them failed.
Between these checks there was an unrelated check, DNS resolution.
If the host and DNS checks both failed, this made it look like
the DNS check was the cause of failed install. Especially if the user
ignored the DNS check in unattended mode, the output was confusing.
Remove the flag and fail directly.
Do the replication agreement check first; fixing this with
ipa-replica-manage del will also remove the host entry.
Also, use the logger for error messages so they appear in the log
file as well as on the console.