Inconsistent error reporting between planners

Here is how enhsp reports parsing errors: in the `result.output.plan` element:

```json
{
"call": "enhsp-2020 -o domain -f problem >> plan",
"output": {
"plan": "Domain parsed\nDistributor not found\n",
},
"output_type": "generic",
"stderr": "bash: cannot set terminal process group (1): Inappropriate ioctl for device\nbash: no job control in this shell\n",
"stdout": "",
}
```

Plus you may notice the stderr polluted by some unrelated internal errors.

The _topk_ planner returns this json body:

```json
{
"Error": "Some error...."
}
```

...that is without the `status` or `result` element.

The `status` element should be mandatory in the returned payload.