[fix] CommandFailedException: ensure error message is always present

If a command with suppressed output failed, CommandFailedException
would be raised with an emptry string as argument, which makes
debugging issues really hard.

In this cases we shall instantiate the exception with the same
message passed to the log.

openwisp_controller/connection/connectors/ssh.py

@@ -170,9 +170,11 @@ def exec_command(
         # abort the operation if any of the command
         # returned with a non-zero exit status
         if exit_status not in exit_codes and raise_unexpected_exit:
-            logger.error('Unexpected exit code: {0}'.format(exit_status))
+            log_message = 'Unexpected exit code: {0}'.format(exit_status)
+            logger.error(log_message)
             message = error if error else output
-            raise CommandFailedException(message)
+            # if message is empty, use log_message
+            raise CommandFailedException(message or log_message)
         return output, exit_status
 
     def update_config(self):  # pragma: no cover

openwisp_controller/connection/tests/test_ssh.py

@@ -54,7 +54,23 @@ def test_connection_failed_command(self, mocked_debug, mocked_info):
                         mock.call('/bin/sh: 1: wrongcommand: not found'),
                         mock.call('Unexpected exit code: 127'),
                     ]
+
+    @mock.patch.object(ssh_logger, 'info')
+    @mock.patch.object(ssh_logger, 'debug')
+    def test_connection_failed_command_suppressed_output(
+        self, mocked_debug, mocked_info
+    ):
+        ckey = self._create_credentials_with_key(port=self.ssh_server.port)
+        dc = self._create_device_connection(credentials=ckey)
+        dc.connector_instance.connect()
+        with self.assertRaises(Exception) as ctx:
+            with mock.patch('logging.Logger.error') as mocked_logger:
+                dc.connector_instance.exec_command(
+                    'rm /thisfilesurelydoesnotexist 2> /dev/null'
                 )
+        log_message = 'Unexpected exit code: 1'
+        mocked_logger.assert_has_calls([mock.call(log_message)])
+        self.assertEqual(str(ctx.exception), log_message)
 
     @mock.patch('scp.SCPClient.putfo')
     def test_connection_upload(self, putfo_mocked):