standalone: Expose sepcific exceptions in debug mode

In the commit dc559dad specific exceptions
has been exposed to caller, but in case of SLAPOS_TEST_DEBUG enabled unusal
exception is raised.

This commit improves the situation so that behaviour is consistent neverthless
the SLAPOS_TEST_DEBUG.

slapos/slap/standalone.py

@@ -861,7 +861,10 @@ class StandaloneSlapOS(object):
         if e.returncode == SLAPGRID_PROMISE_FAIL:
           self._logger.exception('Promise error when running %s', command)
           import pdb; pdb.post_mortem()
-        raise
+        raise SlapOSNodeCommandError({
+            'output': 'No output available in debug mode',
+            'exitstatus': e.returncode,
+        })
     with self.system_supervisor_rpc as supervisor:
       retry = 0
       while True: