Handle SSH connection errors instead of crashing

An unknown hostname, a refused connection or an unreachable server
raise OSError (socket.gaierror, NoValidConnectionsError), which
paramiko.SSHException doesn't cover, so the program crashed with a
traceback. Log the error and exit with code 5 instead.

simple_backup/simple_backup.py

@@ -177,7 +177,14 @@ class Backup:
             self._remote = True
 
         if self._remote:
-            self._ssh = self._ssh_connect(homedir)
+            try:
+                self._ssh = self._ssh_connect(homedir)
+            except OSError as e:
+                # Unknown hostname, connection refused, unreachable host or timeout
+                logger.critical('Can\'t connect to the server %s', self.ssh_host)
+                logger.critical(e)
+
+                return 5
 
             if self._ssh is None:
                 return 5