add proper check for DNS error in list_target_files (#1625)

The elif needed to check to see if the string was in the listing of results of the shell command.  As it was the conditional was just the string which always evaluates to true and was therefore giving a misleading error message.

management/backup.py

@@ -406,7 +406,7 @@ def list_target_files(config):
 				reason = "Provided path {} is invalid.".format(target_path)
 			elif 'Network is unreachable' in listing:
 				reason = "The IP address {} is unreachable.".format(target.hostname)
-			elif 'Could not resolve hostname':
+			elif 'Could not resolve hostname' in listing:
 				reason = "The hostname {} cannot be resolved.".format(target.hostname)
 			else:
 				reason = "Unknown error." \