Fix crash on empty answer to the unknown host key prompt

An empty answer raised IndexError, and running without a terminal
(e.g. from cron) raised EOFError. Treat both as "no", and log why the
connection was aborted.

simple_backup/simple_backup.py

@@ -375,12 +375,18 @@ class Backup:
 
             return ssh
         except UserWarning:
-            k = input(
-                f'Unknown key for host {self.ssh_host}. Continue anyway? (Y/N) ')
+            try:
+                k = input(
+                    f'Unknown key for host {self.ssh_host}. Continue anyway? (Y/N) ')
+            except EOFError:
+                # No terminal available (e.g. cron or systemd)
+                k = ''
 
-            if k[0].upper() == 'Y':
+            if k.strip().upper().startswith('Y'):
                 ssh.set_missing_host_key_policy(paramiko.AutoAddPolicy())
             else:
+                logger.critical('Unknown host key for %s. Connection aborted', self.ssh_host)
+
                 return None
         except paramiko.BadHostKeyException as e:
             logger.critical('Can\'t connect to the server.')