Fix dns_is_enabled() API command to throw exceptions as appropriate

Previously the command returned False whenever an exception occured
which caused installed to continue even when there was a problem with
LDAP connection.

https://fedorahosted.org/freeipa/ticket/5564

Reviewed-By: David Kupka <dkupka@redhat.com>

ipalib/plugins/dns.py

@@ -4266,16 +4266,14 @@ class dns_is_enabled(Command):
         dns_enabled = False
 
         try:
-            ent = ldap.find_entries(filter=self.filter, base_dn=self.base_dn)
-            if len(ent):
-                dns_enabled = True
-        except Exception as e:
-            pass
+            ldap.find_entries(filter=self.filter, base_dn=self.base_dn)
+            dns_enabled = True
+        except errors.EmptyResult:
+            dns_enabled = False
 
         return dict(result=dns_enabled, value=pkey_to_value(None, options))
 
 
-
 @register()
 class dnsconfig(LDAPObject):
     """